Because it was meant to be like "THIS IS YOU, YOU'RE EVIL" but it just made them all think "Damn, this guy is pretty cool, I wanna be like him"

I don't think the movie would have worked if they cast someone who wasn't conventionally attractive though since I think part of the whole point is "even good looking people can be absolute psychopaths."

Though now that unfortunately reminds me of the fact that some REAL LIFE criminals who did things like murder have their own fanboys/fangirls simply because they "look cute." For example Nevada-tan, that one Japanese woman who murdered her boyfriend, and Eric Harris and Dylan Klebold. Committed horrible acts, but still got "simps" because "they look cute."