Personally I don't believe any of these options are quite right. Ideally, most countries are in need of substantial prison reform.
If I had my way, we would adopt more of a nordic model. We can hate them and their actions, and remove them from the population, without cruelty or death. Putting the life and death of citizens in the hands of the government as an option for "punishment" has too much room for abuse or error. And plus, they will die anyway one day. Why let them escape the pain they have caused so easily?
Killing them leaves too much room for error, and lets them get off too easily. Why should they get to die when others have to live with the consequences? Why shouldn't they live, be given aid whether it works or not, and be made to benefit the community they harmed? In a way that still respects them as living beings, of course.
I think it is better to give them life without parole, in addition to intensive therapy. And at the same time, while they are living in a better prison model, they should be made to contribute to society. I have a few ideas on how, and how we could prevent any abuse in that system as well. Plus, if therapy helps, they will live with their demons as punishment. If it doesn't, we can study them and better prevent others from following the same paths.
Breaking cycles of abuse without compromising our own freedoms, and all while finding better ways to function as a whole, is far more important than a (justly held) vendetta. They should never be released. But there are better ways, and more satisfying ends, than giving them an escape route through a needle.