If violence is unethical, then how can it be justified in any context?
That's "begging the question," in that it demands that the opponent accept as a premise the very conclusion that the proponent is arguing towards, namely that violence is by definition unethical.
And frankly, when it comes to questions of morality and/or ethics, the argument that the opponent is hypocritical may not be
logically valid (see the "tu quoque" fallacy
http://www.infidels.org/library/modern/mathew/logic.html#tuquoque), but it certainly carries weight all the same. Fine, speaking from a strictly logical perspective, when some evangelical preacher who routinely condemns both homosexuality, extramarital sex, and prostitution gets caught going at it hammer and tongs with a rent boy, that doesn't necessarily invalidate his arguments. But when he claims that having done so doesn't automatically make him a bad person, when that's been what he's been saying all along as long as it applied to people
other than himself, well, then we have an issue.
Demonstrably failing to practice what you preach is acceptable on debate teams, because the outcome doesn't influence public policy; but when you
are trying to influence public policy, it's perfectly legitimate to question whether the proponent of a certain measure actually conforms to its principles in practice. When a U.S. senator who proposes that citizens be prohibited from carrying firearms in public has a handgun fall out of his clothing and clatter on the Senate floor, you're perfectly justified in dismissing his arguments on the basis that he evidently doesn't believe them himself.