(Actually, if anything, I'm more annoyed by her statement a few lines earlier that "A cry for help is when you say 'help' in a loud voice." I try to have sympathy as I know in context that she's talking about Faith's attempt to murder someone she loves, but that doesn't change the fact that a cry for help really really isn't always saying "help" in a loud voice. Buffy doesn't spend a lot of time saying "help" in a loud voice in S6, or S7, now, does she?)
A lot of fans get upset about Buffy's claim that she "never murdered anyone." It's technically true - I've never heard anyone deny that. But many fans argue that she's separating herself from Faith, and particular Angel, and presenting herself as better - even though she has committed attempted murder, and it's only simple chance that keeps her out of their "club".
But let's look at that line in context.
Angel: "I know Faith did some bad things to you."
Buffy: "You can't possibly know."
Angel: "And you can't possibly know what she's going through."
Buffy: "And of course, you do? - I'm sorry. I can't be in your club. I never murdered anybody."
Angel is the one who separates Faith and himself from Buffy. Not Buffy. Angel. From Buffy's perspective, she came to L.A. to save someone she loves from the woman who has repeatedly tried to rip her life apart with horrifying results - and finds the man she loves helping said woman and rubbing it in Buffy's face that the two of them have more in common with each other than either do with her.
Buffy's making it far too much about herself - understandable given what Faith's done to her, but petty nonetheless. But her statement "I never murdered anyone" is not a judgment of Faith or an attempt to make herself feel superior to Faith. It's a reaction to Angel's claim that she can't understand what Faith is going through - Angel's identification with Faith and his attempt to convince an upset Buffy that she should emphasize with the woman who victimized her. Buffy in this moment does not feel superior because she's not in Angel and Faith's club. She's devastated about being shoved out by her ex-boyfriend in favor of her rival - and furious that she's being shoved out precisely because she has not been evil.
Angel is the one who told Buffy she couldn't "possibly" understand Faith. Angel Angel Angel. All Buffy did was spell out what that meant.
That said - I do think it is probably significant that Buffy phrases this in terms of "murder", and not, say, "evil" or some other word that Buffy doesn't almost almost qualify for. I agree with fans who hate the lack of Buffy-related follow-up to Buffy's stabbing of Faith. But I do think Buffy thought about it. Between Ted and Graduation Part 1, Buffy has had two experiences of believing she killed a fellow human being, only to find out she was mistaken. In both cases (Ted explicitly, GD judging by facial expression and body language), she was deeply shocked and upset by what she had done. And then it turned out that she had been given an out, that the person she had "killed" had survived. In both cases I think she deeply relieved. So when Angel says to her, "you can't possibly know what she's going through," and Buffy in a self-involved moment hears it in terms of her rivalry with Faith over Angel, I wonder if she didn't have a moment where she thought that if Ted or Faith had really been dead she could be in that club, could understand Angel better. So the word she used was "murder" - because I can't help but wonder if for one, petty moment she was jealous that Finch stayed dead and Ted didn't.