I find it to be irrational at surface value but we could work it.

Assuming all of these people are complete strangers we can safely assume that out of the five million, by sheer probability, there will be many, for lack of a better term, utter pieces of shit. I'm talking murderers, rapists, warlords, psychopaths etc.

There's a high chance that out of the five, all individuals would be decent, at least.

But even in the first scenario, if we're working with a purely utilitarian mindset, the number of decent and good human beings greatly outweighs the number of evil human beings. Hence, the net value of your negligence would be deeply in the red.

But that's too black and white. Sometimes bad people do good things and vice versa, and that's not to mention that each person is a vessel for offspring, children that could be the facilitators of genocide or the pioneers of human advancement.

I guess what I'm trying to say is that you don't have enough information to make a useful statement for one over the other in regards to the well-being of other humans, you can kill both or neither. What you can be certain of, however, is the psychological repercussions that may come as a result of your actions in either scenario, you know your current morals and values.

Therefore, you must and most likely will make a decision that will result in the least amount of suffering caused to you from negative emotions such as guilt and trauma. As you'll notice, you've already done this.