In cases like Paul Bernardo where there isn't a scintilla of doubt. Kill him. But add suffering before hand. However killing them mercifully doesn't really punish the crime. And with people in charge of a corrupt system, there is little chance of blind justice.
It's interesting you bring up Bernardo because I have an acquaintance who worked at Millhaven when Bernardo was an inmate there (he's since been moved somewhere else).
From what I have heard first-hand, there is nothing left of Bernardo. The prison system has broke him. He's mentally nothing but an empty husk of a person. He spent decades living in a small cell all by himself 23 hours a day with CCTV monitoring him 24 hours a day and a guard's desk stationed near his cell.
On top of that, any time he did get moved around outside of his cell, he was flanked by guards and he still had to fear for his life because a lone inmate could still get to him if they really wanted to. Indeed, this happened when he was at Kingston, and another inmate beat the living shit out of him.
Another 10 years on the inside, and Paul Bernardo will likely be nothing but a completely frail dotard wandering aimlessly around his tiny living quarters talking to himself and smearing his own faeces all over his face, if he's not there already.
That's a fate worth than death, because at least the gallows would allow him to die before he completely lost whatever dignity he had left.
As for cases "where there isn't a scintilla of doubt" as to one's guilt. we should not be convicting
anyone regardless of their crime unless their guilt is that provable.