At what point does utilitarian philosophy become "moral" or "immoral"?

Imagine there's a kid playing on some train tracks. The train's racing towards him at full speed, you're a meter away from the kid and you have only 3 seconds to act. The reasonable thing to do seems to be to push the kid out of the direction of the train in order to save his/her life. Now, the push, in itself, is an immoral thing to do, as it qualifies as assault, but the kid's alive because of the push. Most of us would agree that the action is justified by the end result in this instance.

What about a subject like taxation? From an anarcho capitalist's point of view, taxation would be immoral as it is the taking of one's labour without the consent of the worker. However, from a socialist's point of view, taxation is moral because the wealth is redistributed to those with a lower income.

With the train scenario, there doesn't appear to be an alternative to pushing the child out of the way. Perhaps you could hold the child's hand and happily skip across the train tracks, but in 3 seconds you're not wanting to take that risk.

Is the taxation scenario comparable to the train scenario? Well, no. There are many alternatives to taxation that would prevent the infringement of one's own right to liberty, so does that make the utilitarianism immoral in this instance? I would say so.

So it appears that utilitarianism's moral basis lies with whether there are alternatives to said scenario, which ultimately determines the morality of the action.

But let's go further... let's say you're stranded on a desert island, and the only thing to eat are rabbits. Now, you have no alternatives, but you know that the killing and consumption of the rabbits is the only way you're going to survive. We can agree that it's probably right to eat the rabbits, but is the act in itself moral? No, because you're ending the life of an animal that wants to live, but you have no alternatives in the goal of survival, so is utilitarianism moral in this instance?

Discuss.