If that was truly the case I would choose neither and abandon utilitarianism for an existentialism/nihilism approach, violently rebelling against both choices - even if in futility. A deontologist would sit back and do nothing, not wanting to participate in any violence as it was morally incorrect.

That's a very deontological response, though. Your decision is that genocide is wrong, and therefore, no amount of genocide prevention is worth engaging in it - a strongly **anti-**utilitarian position. A deontologist is not simply someone who says "Violence is unacceptable", but who says "Rules are inviolable (or near to it)".

Your rule is not "Violence is unacceptable" but "Genocide is unacceptable"; your response is entirely within a deontological framework. When presented with a situation where you have a clear preference of outcome, but in which all decisions to reach the desired outcome violate your deontological moral sense, you choose to prefer action that avoids the violation of your deontological moral sense even if it leads to a worse outcome, in a very anti-utilitarian sense.

But luckily we do not live in such a world, and we always have more than 2 options with how to proceed.

I generally agree. Mostly I bring this up for two reasons:

1. Pointing out that deontology is very much part of most people's moral intuition, including your's as argued on here.

2. That there are no-win scenarios in moral reasoning, short of asserting that you are capable of effecting literally any outcome via any process just by doing it hard enough. In which case, I would kindly ask you to save the world, because it's really shitty as-is.

There may be a million choices instead of two, but there's no guarantee that any of the million will be acceptable in both process and outcome to you. You may have to make a choice, and the choice may be ugly - and even refusing to make a choice is a choice of inaction (or ineffectual action), with all the guilt that implies.