Prototype Inspired Game
I'd recommend at a minimum Supers and Powers for GURPS. Stay away from the Bio-Tech books - those are generally about realistic transhumanism rather than anime-style transformation into a monster. (Which is the problem with using GURPS for this.)
Double-Cross gets very lethal, though what may turn you off there is its focus on relationships and having a humanity system, which is unfitting at best for Prototype and completely out of theme for Guyver.
I think there's rules in the GMG for M&M to make things more lethal. In the corebook, it's possible to simply presume that defeated minions are dead, which allows for humans to be fragile to the point of explosive, but PCs and major adversaries will still generally require concentrated effort and intent to put down. (Which, admittedly, does fit if you want PCs who can regenerate from a single cell.)