Thrax is a pretty scary, intimidating and completely psychopathic villain.
When he emerges from the contaminated egg, the first thing Thrax does is brutally murder a cell on-screen with his deadly left index finger, casually saying "Ow". We see the poor guy rapidly boil and deteriorate as his eyeball bursts into boiling liquid magma, screaming as he bursts into flames. This is the on-screen kids-unfriendly death that sets the tone for the entire film.
For that matter, the Red Death. Compare Ebola to dandruff. For the sake of the argument, Ebola is X times worse. Now imagine something X times worse than Ebola and a man dying of it and you have the Red Death. Not only that, remember Thrax's unveiling of his plan to kill Frank in 48 hours? He's killed several people before by stealing their posterior hypothalamic nuclei, the part of the hypothalamus that controls body temperature, showing off a piece of jewelry with several of these stolen nuclei. The thought of having a sudden, uncontrollable fever that will only go higher and higher and higher as time passes by...
Thrax: Ya see this? This little DNA bead comes from a little girl in Riverside, California. Didn't like to wash her hands. Took me three whole weeks. And this one, nice lady in Detroit, Motown. Six days flat. Then there's this old guy in Philly. I killed him in 72 hours. Yeah, I'm getting better as I go along, baby, but the problem is I never set a record! Until my man Frank, that is. I'm gonna take him down in 48 hours! Get my own chapter in the medical books!
For added horror: that is how some viruses kill in real life. When the invading pathogens become too dangerous, the body eventually reaches its Godzilla Threshold trying to burn them out, causing microthrombi and autoimmune reactions in a phenomenon called a Cytokine Storm, which in a nutshell, is basically the host's insides LIGHTING THEMSELVESON FIRE in a desperate final attack to destroy the germs. If we were to put the Cytokine Storm into human context, it would be like humanity firing all of its thermonuclear/hydrogen weapons just to win the tide against an Alien Invasion that's quickly turning dire. Unfortunately, this is often what ultimately puts the final nail in the coffin for the dying host. Moreover, Thrax's preferred method of killing - by melting the victim cell into slag-like goo - kinda is what a virus does to a living cell: replace its core with the virus DNA until it's overloaded and ruptures itself. In fact, the movie is Lighter and Softer compared to real life, because normally when a cell dies from the virus, it erupts into a swarm of its copies. Agent Smith would be proud.
As mentioned above, one of Thrax's previous victims was a little girl who "didn't like to wash her hands", killed just to get his "own chapter in the medical books", and his only regret was that he didn't kill her fast enough. Given that his modus operandi is to get in, cause all the symptoms of a common cold, and then go for the hypothalamus, it's haunting to imagine what her parents must have gone through...one moment their baby seemingly has a cold, the next she's burning alive, and then suddenly she's gone.
The sheer scale of death Thrax has caused. Killing a handful of humans is bad enough, but the average human body contains as many as 30 trillion cells at any given time and billions of more in microbes that make up the microbiome (the human mouth alone houses more bacteria than the entire human population). And by the film's logic, every single one of those cells is alive with their own personalities and emotions and the microbiomes, while not as prominently featured in the movie, can be synonymous with macroscopic ecosystems we're familiar with. So when a human dies, it's like an entire planet facing total extinction and dying. And Thrax being Thrax, there's no quick explosion like in A New Hope, but rather the planet slowly burning to a crisp. Slow, and agonizing to watch.
Thrax killing Scabies by slashing him open with his claw.
Thrax killing Bruiser and Joe Cramp when they suggest incubating (which would mean he won't be able to break his record).
Thrax:You incubate! I said 48 hours, I'm gonna make my deadline. (Stabs a goon in the eye with his claw and bears down on it, causing said goon to whimper in agony. Thrax burns down his hideout, killing his thugs in the process.) Medical books aren't written about losers!
Even worse, had he not been stopped by the fake eyelash he and Ozzy were battling on, he was planning on going after Shane next and breaking his record of 48 hours. The Slasher Smile on his face as he says this makes it even worse.
Thrax:(seemingly impaling Ozzy with his trigger finger) Can you feel the heat, Jones? (chuckles evilly) Too bad you won't be here to see me break my record when I take down Frank's pretty little girl.
