The whole "Buffy is sad because she's no longer in heaven" arc was kind of a key one for me; it used the whole thing to run a series-long arc giving one of the most detailed metaphorical dissections of serious clinical depression I've seen in a series. The reasons for her depression are clearly impossible, but the increasingly self-destructive, self-harming behaviour Buffy engages in are absolutely real. The suicide attempt in the musical episode is really important, as it's the point wherethe audience explicitly sees that Buffy isn't who she was in season 5; the selfless hero you refer to has gone. Which is what depression does to people - they go from being one person, and change utterly, and not for the better.
She knows dancing will kill her; they establish this early on as the reason the demon has to be stopped.
Buffy tells her friends that they've pulled her out of heaven, and back onto Earth, where every moment is agony by comparison. She turns to them, and sings/screams "Give me something to sing about! PLEASE!!" Here, she is obviously asking them "Give me a reason to be happy! Give me some possibility that things will get better!" This is quite important as it's the first time she's voiced her depression, and one of the key traits of clinical depression is the belief that the depression will last forever. Sufferers of actual clinical depression tend to be unable to envisage life without depression - it feels like being unhappy all the time is just the normal state of things. (It als bears stating - it's not; if you're a person who genuinely believes that life is pretty much miserable all the time, then there is a high probability you are suffering from some kind of clinical depression, and you should seek counselling. Thus speaketh the voice of a man with sixteen years of bitter experience.)
When none of them can say anything, well - what's the point in going on?
So she doesn't fight the demon, or do any of the heroic things she would have done in earlier seasons.
She says nothing more, and starts frenziedly dancing, knowing she will die. She has knowledge of the effects of the dance; she could choose not to. When her friends remain silent, she makes her choice.
What further proof of
intent do you require?
It's also noticable that Dawn no longer matters, unlike season 5. Buffy's depression has left her feeling isolated, alone, and as though nothing else matters.
That's kind of the point of this episode. She
is selfish now - she can't see beyond her own pain. Her friends put their pain at missing her ahead of her happiness (in heaven); why should she put anyone's pain ahead of hers? The fact that Buffy tries to kill herself illustrates just how broken she is. She knows what the dance will do, but she doesn't care, because all that matters is making the pain stop. Suicide is only a cowardly, selfish act from the outside, somewhere Buffy doesn't care about at that moment. Her selfishness is then further illustrated by the fact she starts a "relationship" with Spike, because feeling anything has become better than nothing... ignoring the fact that Spike's feelings are geniune, and to use him in such a way (even if he wants it - which he does) is appallingly immoral from her side. Her time with him over series 6 is one long bout of self-harm, using Spike as a tool to hurt herself, not even remotely caring about what that behaviour does to him. Which, I think, is a pretty good argument against the "always selfless" Buffy you refer to. She stopped being that person a long way back - the whole story of the season arc is about how she overcomes her depression and her selfishness to get back to the hero she was.
The whole thing is extremely dark and involves mature storytelling in the best possible sense, in that it's about complex adult emotions that exist beyond simpel "love = good, hate = evil" dynamics.
It's one of the single darkest moments in the whole series, but because they hid it amongst cheery song-and-dance numbers, and made the method of suicide so far removed from anything possible in the real world, people just kind of ignored it. It's not Buffy's lowest point, but it's certainly one of the bleakest moments.
But it is that her life is so terrible that she can't stand it! The only way to get to paradise is to die, so that's what she chooses.
Like I say, Buffy was a great series - you could read the scene multiple ways.