If you ask me, I think that people will do what they do regardless of the consequences if they have mental illness issues. Sometimes you can just tell that some people do not care about the consequences of their actions.
If you want to know what many regular people think on this matter, there are online debates you can check out. Debates on the death penalty are pretty much even on either side, which is why it's hard to say either way if it's going to work in each case.
These things are spooky:
1 - http://www.debate.org/opinions/does-the-death-penalty-deter-crime