Where the verdict is 100% certain, taxpayers have far more important things to spend our money on than keeping a vicious murderer alive, just to keep the opportunity to escape open to them. As I understand it, it is currently very expensive to execute in the USA: needless to say, this is a problem with their organisation and it doesn't have to be this way.
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.