Because villains who want to do good but won't sign are FAR more dangerous than mass murdering psychopaths who drink baby blood(well, I'm sure he's gotten some spatter on him at some point at least. My mind's stuck on the CW4 parody. If you didn't read it then DO.. great stuff). Duh! :)
Hopefully all this Negative Zone prison stuff is a big hologram and they're concocting something, I don't know, HEROIC? out there? If these guys are commiting suicide over there then it's so far out that there's no other good way to justify it. The REAL villains should be over there, like you say.