Every wrong act is wrong because it wrongs someone or something. Say that an act is fundamentally self-wronging provided that it is wrong because it wrongs oneself. It's controversial whether there are fundamentally self-wronging acts, but I think there are. However, attempted murder (as long as it's not attempted suicide) is not a self-wronging act. But now imagine that Bob is the only contingent being in existence, and Bob attempts to murder someone else (of course, to do that he will presumably have to have a false belief that there is another contingent being). Bob commits attempted murder, which is not a fundamentally self-wronging act. Hence it wrongs someone or something other than himself. Only concrete beings can be wronged. So there is a concrete being other than Bob. Since Bob is the only contingent being in existence, there is a concrete necessary being.