The rational answer is because nothing in life is certain. Someone who decides to shoot and bomb and kill 77 people could decide to do so again. Certainly the odds seem much higher for that person repeating than it is for a random individual to do the same thing.
No science we have can say a person won't do the same thing again.
The emotional answer is that some people serve no practical purpose. I don't understand why society would care so much about the life of someone who values life so little that they can repeatedly kill dozens of people.
We have no problem as a society putting down a deranged dog, even though we could totally lock it up in a cage and let it live out it's days there. Heck, maybe it won't be deranged when it is 9 years old.
Let's consider this hypothetical: I am just really curious what it is like to kill someone, so I go out and randomly kill someone you love very dearly. After I do it, it turns out I don't like killing at all and never want to do it again. The rest of my life will be devoted to peace and love.
