In general, the primary problem with such behavior is that if lots of people do this sort of thing society falls apart. Thus, there's a bit of a prisoner's dilemma here. So any logic favoring "cooperate" more or less applies here.

Note also that for many people they would probably see this as wrong simply because humans have a tendency to see a major distinction between action and inaction. Action that results in bad things is seen as much worse than inaction that results in bad things. Thus, the death of the guard seems "bad" to most people. This is essentially the same issue that shows up in how people answer the trolley problem.

So, let's change the question: If there's no substantial chance of killing the guard should one do it?