Peter, like you, I used to be in favour of capital punishment. That was until I realized how many innoncent people had been convicted of serious crimes.
The whole premise of the criminal justice system (ok, in the US, but really not that different here) is that it is better for 100 guilty people to go free than it is for 1 innocent person to be convicted. Based on your argument above, you would prefer that 100 innocent people be executed, rather than 1 guilty person be incarcerated in prison (that's right, not go free, but be sent to prison). And you can honestly say you are ok with this?
Of course you would not feel the same way if this were your friend, or your parent/sibling/child or yourself and you believed that they/you were innocent. To claim otherwise, unless you have already been in put in that position (which you couldn't have been put in in Canada) is absolutely ridiculous.
Let me hear you speak of the "greater good" as they are about to give you a lethal injection, or turn on the electric chair. I daresay your last words would be closer to "I DIDN'T DO IT! I'M INNOCENT! YOU ARE MAKING A MISTAKE!"