I'll weigh in.
Kerrigan is frighteningly powerful, for an overclocked human-Zerg hybrid. She can pulverize hordes of lesser foes, and probably go toe to toe with some fairly scary stuff, and we're speaking about the Starcraft universe, which contains things which are scarier than the Zerg. No one is doubting Kerrigan's ability, her power, or her competence.
Now, let's face the facts about the Swarmlord. Forget about his tabletop performance, because they do not represent the fluff. The stats change from book to book, but the fluff remains fairly consistent about Hive Tyrants, and the Swarmlord as well: they are not like a Termagant when their ISP to the Hive Mind cuts out. They are -Avatars- of Hive Mind aspects, in the same way that Greater Daemons are like miniature versions of their patron deity. The key difference is, Chaos Essence (The One True Chaos which, for all intents and purposes, could simply be the Warp itself) is divided into four key beings, which embody different "flavors" of Chaos ... almost like primary colors. If a Greater Daemon could exist indefinitely, and survive without the Warp in isolation, it may not "know" as much as a Chaos God, who knows everything their little baby Greater Daemons know, because they're literally PIECES of the deity in question, but ... it would still have the same core "personality" of that Chaos God. The same moral compass, likes, and dislikes.
Hive Tyrants? They are "pieces" of the Hive Mind, with their own "section" of life experiences. Their core motivations, desires, drive, and goals, however? They're identical. Two Hive Tyrants may have fought completely different species, but both could "agree", hypothetically, on the mutual goal of total universal lifeform assimilation. They may differ on approaches, methods, and experience, but they are a case of nature-versus-nurture, with the nurture part conveniently done away with: they are all the same Tyrant, upon creation. Their core programming is the same. The Swarmlord is simply the most "senior" of these beings that we know of, having fought and defeated "countless" civilizations over the course of AEONS. If you remove the Hive Mind, the Swarmlord is still the Hive Mind in miniature form, with less aggregate experience to draw upon. Previously, with Hive Mind access, he could browse his entire video folder of memories detailing all the hundreds of THOUSANDS of civilizations he has PERSONALLY overseen the destruction of... and if, for some weird reason, he is somehow shocked to find a foe for which he could think of no corellations, he could just browse all the COLLECTIVE Hive Mind files in the Cloud.
Take away his Cloud? Okay. He's still horrifically psychically powerful, ancient beyond mortal understanding (probably billions of years old? The Codex says they have consumed dozens of galaxies, and Tyranids don't travel very fast over interstellar distances, so that much covered ground would have taken him awhile..), has all of his OWN collective memories to draw upon, still stifles Psykers (and even people who AREN'T psychic..), actually has GREATER chances of driving Psychics insane if they happen to be very powerful (and therefore, very sensitive), and still happens to be a 24-foot tall, six-limbed monstrosity with swords that devour your ******* soul if he so much as KNICKS you.
Let's talk about his speed. He is 24' tall, and moves faster than a Genestealer. Okay? A Genestealer. A human being, military-trained and in good physical condition, represents a core baseline of Strength and Toughness 3. This does not represent a 57 year-old man with diabetes who runs a Prometheum fuel relay station, okay? This is someone who could be a Cadian, and Cadians make Schwarzeneggar look like a wuss. This is Strength 3/Toughness 3, where a bullet can, and frequently does, one-shot you. Bullets are scary.
A Space Marine represents a Strength and Toughness of 4. A Space Marine gains his S/T bonus from being directly neurologically interfaced with a nuclear-powered suit of armor. He is 9' tall, weighs around 600 pounds in full battle dress, and has about 17 more organs than you do. His muscles are powerful enough to uproot a tree -- not a sappling, a TREE -- and smash you with the trunk. He is the fusion of any THREE of the biggest, meanest lumberjacks that you could find. He is Strength, Toughness, and Initiative 4.
With HUNDREDS OF YEARS OF TRAINING, EXPERIENCE, AND EQUIPMENT INVESTMENT, a Space Marine CAPTAIN could rock Initiative 5, and Weapon Skill 5, fully 2 points above a regular human, and one above a Super-Man In Fusion-Powered Super-Armor With Super-Organs. He would be so fast a human would strain to see the movement that killed them. A Space Marine would be a blur, but at least you'd see something coming -- you'd just have no hope of stopping the killing blow.
A bog-standard Genestealer is more SKILLED in combat by 2 points, than the scariest Tactical Marine, who makes a human who has spent their entire life training and fighting look like a novice, with the physique (by comparison) of a child. You're 2 feet shorter than me, and you're slow and weak. Don't embarrass yourself, says the Space Marine inductee to the six-time worldwide Cadian heavyweight boxing champion. Then, that Space Marine meets a Genestealer in close combat, and HE does not have time to see a Genestealer's killing blow... and even if he could, he can't hold a candle to his Captain in a one-on-one fistfight, and this SINGLE (please do not swear) GENESTEALER WHO IS LIKE EVERY OTHER GENESTEALER is better in melee than the scariest Space Marine a Chapter has ever produced, so far as physical stats are concerned.
A Genestealer moves so fast, it makes a Space Marine feel like a human trying to fight a Space Marine Captain by himself.
The Swarmlord? He has Weapon Skill 9. Okay? 9. Consider that number. That number does not mean "three times better than a trained human at fighting." Weaponskill 5 on a Space Marine Captain means he has studied nothing but WAR for probably half a millenia. Five-hundred years of fighting, reading about fighting, praying about fighting, being tested by Chaplains on how hard you've been praying about fighting, fighting about who to fight, sleeping and REMEMBERING fights, because Space Marines don't dream, but if they did, it would also probably be about fighting, or else that one time when they were twelve and maybe thought girls were kind of cool.
He's Weaponskill 5 after about five-hundred years of that, and Space Marine steroids + equipment.
Weaponskill 7? That's an Archon. He was already scary because he was an Eldar, but he's eaten enough souls to live to see three-thousand, and killing things is his favorite thing in the whole, wide universe. Weaponskill 7 isn't even imaginable. You arrive at Weaponskill 8, and find Asdrubael Vect chilling there, pushing 14,000+ years old. His skill, Ancient Nemesis, says that he has lived for so long, that he knows precisely how to kill everything that walks, crawls, swims, and poops itself.
Weaponskill 9? Lilith Hesperax is supposedly around here, but she's like the most skilled toothpick that ever lived, probably supernaturally good, very old, and naturally gifted, in the same way that some people are simply prodigies -- she is the only mortal creature who exists at this rank, in a universe as scary as 40K.
And then there's the Swarmlord -- good enough to fight Lilith Hesperax to a standstill, if he was stuck in an Eldar body.
Luckily, though, he's not -- he's still chilling in his 25', six-limbed murder-machine of a body, which is also as physically tough as a tank in terms of how difficult it is to damage, and then further augmented by psychic powers which allow him to shrug off ridiculous wounds, AND offensive psychic powers, AND psychic defenses, AND instant-death swords, AND speed on par with a Genestealer, AND melee skills on par with Lilith Hesperax, and bested only by, LITERALLY, an EMBODIMENT OF THE ESSENCE OF KILLING THINGS.
Now, I'm not saying Kerrigan isn't cool, or powerful. I'm saying, I feel I'm being generous if I say she's Weaponskill 9... and even if she was, she's still just "Herself" without the Overmind ... she has only her own experiences, her own very short lifetime of memories, and her very limited number of warfare engagements, compared to something which is physically, mentally, and psychically designed to be superior to the very best that a DOZEN COLLECTIVE GALAXIES OF CIVILIZATIONS (or more... the Codex says "at least"..)could offer. It has no wants, no hopes, no desires, no fears. It embodies hunger, and every atom in its body is unified in the purpose of bloody evolution.
Kerrigan is, quite simply, too human to compare. She would die. She would die hard. She would die screaming. And she would end up in the same place as the billions of other supposed bad-asses who decided they could step up to the plate -- actually, a different place. Different places... like, a piece over here, and a piece over there...
"Did you see what happened to Kerrigan? I heard she got Marneus Calgared."
