When you have 1,000 people who want to kill you, how many can you kill before allowing yourself to be killed, because, you know, it's "disproportionate"?
That is an excellent question.
Anyone who is in the process of knowingly and willingly attempting murder has voluntarily given up the right for the morality of their threat elimination to be ascertained.
If one were you draw a moral dilemma, I believe the appropriate example would be a murderer hiding behind 1000 human shields. In such a scenerio, the question can be posed whether there is any moral justification to kill an innocent person to save yourself. On the other side of the equation, it's not just you at risk, but an entire population whom you are charged with protecting.
This is the question which ethical conundrum books were written about. However, there is a number in which most proponents of non interventionism in the trolley problem would switch over to advocating for sacrificing the life of one to save the lives of many. What that number is, I do not know. Neither does the Geneva convention or the ICC. The only thing they do agree is that a country's first responsibility is towards its citizens whom it is charged with protecting, and the elimination of a threat towards its citizens is justified even when it inevitably comes at a civilian cost.