It helps make the world a safer place.
No, life is sacred, God decides when to make people die; by using the death penalty, your destroying gods creation
you can only commit homocide by killing someone, so by giving them the death penalty you become just as bad as them. Another reason is that keeping them in prison would be worse for them as there suffering would not be over quickly, but if you killed them you would be giving them an easy way out. This is unless they're mental, if they are they need to be isolated so they don't do anything dangerous and not killed because people shouldn't decide to kill others as this is sinful and can affect you and others morally even if you don't think it will beforehand.