Lets see what sounds worse, a 19yo spending the next 50 years in prison, vs. shooting him in the head?

If you want to live by the 'Eye for an Eye' mentality, I might suggest you move to some 3rd World country in the Middle East.  As those place might serve the justice you believe in.  Here in America though, we have a system that people have to go through based on these things called "rights".  You should look into those....there are quite a few of them.