It would look bad if he were caught lying about his broken nose. But we cannot assume without x-rays either way if it was broken or not. This case seems more of like a disparity of force issue. According to Zimmerman's story, he could not get away from Trayvon, and was in fear for his life. If this indeed is how the story went, he could have not even suffered a cut/bruise/broken nose, and still justifiably shot him due to such. It's sort of like when I asked Massad Ayoob in the gate about if a group is telling me they're going to do harm to me, yet have not yet, and I have no reasonable way to escape the situation, the shoot would be defensible. Even if that shot was taken before the "beatdown" even began.