In this situation, you seem to have no capacity to do right. You are either killing many or you are killing one. Neither is good.
If you are directly causally responsible for the deaths in both options, and not you simply internalizing making a decision that actually would be made by forces outside of yourself, I would propose that the correct option is to postpone making the choice indefinitely. Or, if at all possible, to remove yourself from the situation, preventing either outcome.
It may simply be that someone would die without your action at all. This is not your moral responsibility. You can intervene yourself to try to save them, but that may be beyond your individual power, and you shouldn't beat yourself up over it.
However, if you're asking if you should murder a whole bunch of people to save someone important to you, the answer is clearly "no, of course you shouldn't", and if you're asking if you should murder someone you care about in order to save a whole bunch of people, the answer is clearly "no, of course you shouldn't".