Because I consider it to be a just punishment.
I don't believe there is any value in rehabilitating someone who commits the most heinous of crimes. How can they ever repay society for the rape and murder of an infant?
Well that depends entirely on whether you want to get "value" from someone, and how "value" is actually measured. I don't think that we should get "value" from anyone who is incarcerated.
So the only possible reason to place them in prison is to torture them, which I do not approve of. It is more humane for them to just be hung and more just for society and the victims.
Again, it depends what you're referring to by torture. I would consider life imprisonment sufficient for heinous crimes. You lose your liberty, your family, your friends, your life. That's enough.
I don't agree with the deterrence angle, although the source posted earlier about how there are less murders in the north east USA is fairly chuckle-worthy. The north east of the USA is the richest region of the country with the lowest ethnic diversity. Of course it has a lower murder rate.
I think (I don't have the source) that it has been shown that capital punishment does nothing to deter criminals. In most cases, crimes like these are spur of the moment emotional decisions, and have no prior motive, therefore meaning that deterrents are irrelevant. In the case of pre-meditated crimes, anyone who plans a crime such as these doesn't care about the consequences or is fully aware of them.
Why do you say of course it has a lower murder rate? I'm guessing you're referring to the evidence that says that immigration and ethnic diversity cause economic difficulties, which results in higher crime?
Capital punishment is absolute hypocrisy.