Lawyers, judges, and juries are all fallible people. If you lock someone like Hurricane Carter up for life, you can, when he is proven innocent, say "Ooops!" and let him go. That is my chief objection to capital punishment.
My other objection is that it's much too nice for most killers. A child killer either does time in solitary, or risks his life among his fellow inmates, who will be only too happy to beat the tar out of him. This goes double for a child molester.
Charles Manson's women weren't executed. Instead they have to live, every day with the knowledge that they followed a lunatic, and killed innocent people. They aren't happy about it. If Timothy McVeigh had been allowed to live, he might have come to a similar conclusion. McVeigh, incidentally HATED being in prison, and was happy to be executed. Which is a great reason to let him live!