Frustration at people's brooding attitudes and the 'emo' culture. The singer gets so frustrated at people constantly going to her to moan that she says 'if you wanna, die why don't you just die then?' - reasoning that no one will miss them for their depressed, wet-blanket personalities.
She loses all sympathy at their pretend depression, and is sick of pretending to care. They say they want to die but never come close to going through with it. So she exposes them by egging them on.
It hides a lighter message: 'Live with all your might'