While I understand her reasoning for wanting to kill Faith, Buffy acted out of grief and with vengeance.
But Buffy wasn't acting out of vengeance or grief. Those may have been contributing factors but not primary motivations. If she wanted revenge she could have gone after her at any moment, Angels impending fate put some fire under her ass to do what had to be done from the start. It was about saving Angel, if Angel had died then yes I would agree it was about revenge.
What she did made her no different than Faith herself, when you get right down to it
Well if you don't take into account the stuff like circumstance, motivation and the victim then I guess they are the same.
There are many contributing factors to every case which have to be examined individually; this is even how it is done in a court of law.
Buffy was killing a murderer to save an innocent man who she loved; Faith was attempting to poison an innocent man in a conspiracy to kill thousands of people, and that was just one of her crimes, there also the fact that was a renegade killer who could not be held by any jail, plus the only people who could handle her (the council) were doing nothing.
But then again, Faith was evil at the time, and some could say that Buffy killing her would be serving the greater good.
But what other choice was there? Buffy could have simply beat Faith into unconsciousness and then handed her over to the Council in the past. But at the time with the circumstances (Angel/Graduation) that wasn't an option unless she wanted to let Angel die and just kill Faith later in the battle.
Plus I still call what she did defense, she is deciding Angels life is worth more than Faiths, but so does everyone who kills in self-defense, they are deciding that their (or someone else's) life is worth more than the person trying to end it. The only difference is here it's not a split-second decision and appears more like premeditated murder.
Although I could agree that it was a wrong act but the right choice. Like say you have the choice of killing a dog as oppose to not killing it. One is a right and wrong act making it an easy right and wrong choice. But say you then had to choose between killing a dog or a person? In this case killing the dog would be the right choice, but still a wrong act.