Ehh, I don't know. The sides of the issue make it hard to come to a definitive decision.
Pro's: It completely understandable that Armstrong would finally say "fuck it" after being accused of taking drugs for 10 plus years straight, and proving himself innocent again and again. The USADA is, in effect, hunting him specifically, he is retired, and having already proven himself innocent in the past. Yet, they are coming after him anyway, with some rather suspect evidence, in that it is largely verbal accusations from people who may or may not have motives to see him "fall".
Con's: Armstrong could be quitting to avoid being proven guilty. Quitting is not in his character, and usually innocent people don't back down from accusations. He was competing against known drug users and still won more times than any of them, that does look suspicious. The evidence, if true, and verfied, could be damning.
Either way, he beat cancer, and won titles after that. Even if he was using drugs, he is still one of the top athletes in the world. ( bearing in mind that drugs do not turn you into superman, they merely extend your own personal limitations, and in cycling while that would be a huge advantage, it would not be enough to give you a clear cut win, as evidenced by the drug users that still placed second to Armstrong)