The police and justice system will never be infallible, no matter what resources we throw at it. So with the death penalty, you will always execute a number of innocent people. Quite a number if the US system is anything to go by. I don't think that's worth it, when an alternative exists that keeps society just as safe, ie prison.
No, I can't be 100% certain Bernardo won't get out. But if we're crazy enough to let him out, we deserve what we get. Tightening up the parole system is something I have no problem with. That is where more resources should be allocated, so people who do get out on parole are properly supervised.
I'm not clear what you mean in your last paragraph?
So you know where I'm coming from: I'm against the death penalty. Main reason is moral - you don't condemn murder by killing. It just brutalizes society. But also, killing innocents discredits the whole system, IMO, even if you do believe in an eye for an eye.