As with all life, we are driven biologically to value two things above everything else - our own life and our offsprings' lives.
Then, since life is basically considered the highest-valued thing in existence, depriving someone of life without very good cause is generally considered to be the most heinous of crimes because there is no way that it can be undone and there is no way to make any kind of amends for it (as there can be in other crimes which can't be undone, like rape)
What I am asking you is why. We can't blame biology - certainly biology causes these emotions to arise, but we as a a species are quite entitled to make use of our critical faculties and trump our instincts.
It's important to know why murder is bad. As a time saving exercise I'm going to propose my understanding of why murder is bad and you can tell me whether you agree or not. I think it's bad because it is the act of elimination of human liberty without, as you say, good cause.
Now, to my mind, locking a man in a square cell and jailing him until death is not just a constraint on his liberty, it amounts to an elimination of his liberty (with good cause).
The key point is that we can't just say "well then we'll give him resources to exercise liberty" because it is dubious to what degree that can be achieved without contradicting the legal basis of imprisonment. It's likely that if you do apportion him any real liberty, you are diminishing the state's capacity for punitive redress. That's a whole another topic.
So the removal of such a man's liberty is a necessity either way. All I am saying is that (ignoring any complicating factors like risk of error), that individual ought to have his liberty extinguished more humanely, not dangled above society light a caught mouse.
I do oppose the Death Penalty-on grounds of risk that the judgement of an individual's guilt is erroneous or misguided. But I happen to think it's quite sad that we have to find more cruel ways of abolishing an individual's liberty, and sadly we here in Ireland do it particularly maliciously, as the Inspector of Prison's reports have attested.