The worst people in life are those who would see the world bend to their moral beliefs in spite of the detrimental effect it has on everyone else; keeping a mass murderer alive because you think killing is bad is moral narcissism. Saving lives and keeping people safe should never be at conflict with "doing the right thing".
Never try to contain the threat, always neutralise it. How you personally feel about killing should never be prioritised over the need to do so; people will die if you don't.
Look at how many people died to capture these villains once. By refusing to kill them, you are inadvertently declaring their lives to be more valuable than the hundreds they've killed.
God, I really do hate people who do this shit.