As isn't death the easy way out anyway? He'll feel nothing and just go to hell (if you believe in a hell). If you really want to see him suffering, isn't knowing he'll spend most of his remaining life in prison, where he'll have plenty of time to rethink and regret his deed, the much better way?
So I don't see why opposing the death penalty means you want the perpetrator to suffer less. He'll certainly suffer enough.