I don't know if he got any taint or not, but if he did, it was probably of the psychological variety rather than the metaphysical variety.  Nothing magical about it, just that it's easy to get into the habit of solving your problems and making your points with violence.  Doing it and having it work just encourages you to do it again.

Before the party, Harry mentions that he was going out to "first day in the prison yard". The #1 rule of the prison yard is "don't show fear". In fact, your best bet for that is to find the biggest, baddest prisoner and beat  the ever-loving hell out of him to establish your credentials.

In the Winter Court, no such credentials would be awarded to a Knight who showed any sort of mercy. To quote Vince McMahon here, "ruthless aggression" is the name of the game for them and the only thing they respond to. Harry understood this, and acted accordingly. His first "kill" was on Mab's orders and as such, did nothing for him. The second was his way of establishing his identity and establishing the ground rules for how he was going to deal with the Sidhe from then on out. It was pre-meditated, but was not outside the realm of "standard behavior" for ranking nobles of the Winter Court. He understood this, and its necessity, so no taint or corruption would apply to him afterward. He didn't want to do it, but he had to, and that's the crucial difference.