system
Given the following post, respond as if you're commenting on Reddit:
human
[Resident Evil] Why do the villains keep intentionally turning themselves into horrible monsters?

Specifically I'm thinking of Saddler but there are numerous other examples of this trope. At some point the villain decides they need to "power up" and trigger some form of violent body horror mutation to kill the heroes. Aside from the obvious desire to kill the protagonists, why in hell would anyone do this? It seems incredibly unlikely these things are reversable, and it doesn't seem like it's worth it to spend the rest of your life as a monstrous crab/insect thing with a giant eyeball forever blocking your trachea. 

What's weirder is that some characters who end up turning into monsters (Gionne, the arms dealer in V with the absolutely hilarious voice acting) react normally with fear and confusion, but other times it's Alexia Ashford over here turning into an ant queen that certainly isn't going to be able to move from her chosen position...in a maintenance corridor of an exploding Arctic base. Even if these people win they've inflicted a fate worse than death on themselves, so why do they keep doing it?
gpt
In the case of Saddler, who clearly has the most advanced form of the Plaga, it's not entirely clear whether or not changing back is impossible for him.

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I mean, the dude already has a giant ass tentacle available to him in his human form, can change his arms from normal to tentacle then back, and otherwise have some limited transformation abilities separate from his final boss mode.

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For all we know, if he had killed Leon, he could have shed his giant monster form and resumed being humanoid. The Las Plagas as a whole is a lot more versatile than other viruses/parasites in the series.