I would be willing to die a quick and painless death for a stranger. Sure, I may not know them but that doesn't mean they deserve to die a horrible and painful death.
Frankly, I don't believe it's fair for people to argue that they have a right to live anymore than a stranger merely because you don't know them. A stranger (not a creeper) is merely a person not known. If you think about it, much of the universe is not known or understood by us and if we allowed things unknown to us to be sacrificed so that we may live, then we may not know the consequences.
Best case scenario is that you know or get to know the other person before making a decision. Otherwise, I would be willing to gamble my life for theirs with the hope that they may aspire to do something important. Maybe my death will inspire them to be great. I would find it very difficult to continue living, having the weight of guilt holding me back all the time.