Sure, if you start from utilitarianism you can find reasons to oppose murder, although that's not the same as justifying the abhorrence of murder. But utilitarianism assumes we ought to be happy. It's true that we actually want to be happy, but how to go from that statement of fact to inferring that we're morally in the right in having that desire?
And sacrifice of the minority is indeed a problem for utilitarians: What if maximizing happiness for the majority requires the unhappiness of the minority? In that case, utilitarianism says we should use the minority's suffering as a means to a greater end.
Utilitarianism is about calculating means and ends, which is a far cry from justifying the abhorrence we still have for the act of murder. Again, we seem beholden to something other than mere rational calculation.