I haven't read the whole topic but the answer to the question is very easy and simple for any objectivist worthy of this name. Perhaps what I'm going to say has already been said.
By definition, moral choice, good and evil, presuppose values.
Values presuppose life.
Thus, destroying someone's life is against life, so against all values, in general.
No need of God.
Actually the idea of God can lead you easily to murder. Believing in God is based on faith, revelation. This means giving up (at least partially) his reason and this means that this kind of person can accept an idea that has no logical or rational justification, on the basis of a feeling, a desire, a revelation. On this basis, you can be led to murder. Or not. That's just chance, depending of your current desire, according to what your God is supposed to say, etc.