This is the same argument that's frequently used against victims of rape and other sorts of assault - used to deny them justice, on no better grounds than that their own past behavior has been merely human, and thus fallen short of whatever arbitrarily absurd standard of impossible purity that unsympathetic interlocutors choose to establish - and to re-establish, as often as necessary, in order to make absolutely certain that the standard cannot be met.
I wouldn't find that a terribly comfortable place to stand. That you seem to find it congenial gives me to wonder about the quality of your own character. Ordinarily, of course, such doubts would be unworthy of this sort of discussion. But since you've found it meet to broach the subject, why shouldn't we start the inspection of character with you? Clearly you feel yourself above reproach in this regard, so I'm not sure what reason you could possibly have to demur, when I require that you prove, to my satisfaction, your standing to argue as you do on this subject.
The debate is to determine what is right, just, fair, and proportionate.
Fairness in justice does, in fact, depend on the character and behaviour of the assasinators and wider social group, not just the target in isolation from them.
If you want to argue against those principles, that's fine obviously, but argumentum ad hominem fallacy does not apply where the character and behaviour of the debators is properly relevant.
(Character and behaviour are also relevant to the question of estimating how much to believe, when people are acting as witnesses rather than debators.)