I used to think the same until I met a forensic psycologist who explained that we need to better understand how and why these scumbags do what they do.
Then we can pre-empt their actions and prevent these things happening.

The death penalty is too late as the crime has already been committed.

While I agree that they absolutely deserve to die in the most horrendous way possible, it wont save a single person.

Death isn't a deterrent to these people.