I have no problem philosophically with the death penalty, and used to support it, however, I am now against it for a few reasons.
First, it is obviously not a deterrent to criminals. Have you ever heard of a criminal who plans a crime and thinks they're going to get caught? Of course not. With the exception of treason, capital punishment only came into play for premeditated murder or murder committed during the commission of another (planned) crime, e.g. killing a cop during a robbery.
Second, there is the possibility of wrongful conviction - nothing more need be said.
Third, I wish I could find the source, but I recall reading years ago that, prior to the abolition of the death penalty in Canada, only one-third of those originally charged with capital murder were actually convicted of it. The other two-thirds were either acquitted or convicted of lesser offences due to plea bargains or juries choosing such an option. And, of those convicted of capital murder, only one-third were actually executed. This group consisted predominantly of visible and linguistic minorities. White, Canadian-born murderers were far less likely to be hanged than immigrants or non-whites.
Fourth, juries are loath to convict people of capital crimes without proof beyond all doubt, rather than beyond a reasonable doubt. Such absolute proof is quite rare. Following the abolition of the death penalty, the conviction rate of those charged with first degree murder rose from one-third to three-quarters.
However, I believe that a life sentence should be just that - no parole EVER. And. I wouldn't take any means at all to prevent suicide by convicted murderers. The only exception would be the Olsons, Bernardos, etc. I would wait a few years to eliminate the possibility of a wrongful conviction, and then start a regime of systematic torture of the bastards, while the others watched.
On a somewhat related matter, I have never understood why there has always been a distinction between first and second degree murder. I fail to see why someone who kills on the spur of the moment in a fit of rage is less of a criminal than someone who commits a planned murder or a murder during another planned crime. In fact, if protection of society is a factor, someone who plans a murder often has a damn good reason for killing the victim and is unlikely to ever kill again. However, someone who has a deadly, violent temper will always be dangerous.