A Utilitarian would argue against your point by pointing out that the utility created by the revenge murder is less than the utility created by not doing so. With the revenge murder you add another corpse to the pile and probably end up removing yourself from the utility pool. Without it, perhaps the rapist-murderer adds something of value to the world before their death, and you don't remove yourself from the utility pool.

Utilitarianism is simplistic, but then again, so is "I'd feel morally obligated to do it, and I'd be a coward if I did otherwise." Point being, this is not a hard point to argue against, using almost any philosophical framework created in the last couple thousand years.