Ok so, there's this person who talks like and is meant to be perceived as an edgy teenager, he doesn't do much, and the people who aren't very close to him think he's very laid back. This character's the sort of person who's trying too hard to be emo, his dialogue suggests it, when a bully tries to start a fight with him, he laughs his head off, he says he could 'destroy' them all and nobody takes him seriously.
But then he gets in a fight, not the kind of fight a bully, mugger, or even gangster would start, but a firefight, guns blazing, school shooting. The villains are attacking, those people showing up could legitimately end in him and two dozen other people being murdered.
And all of that talk where he could destroy the people who are picking on him? All of those times he says he's a walking death threat? He actually is, he proves he could actually kill every single person in the school in fifteen minutes.
Plot twist, he's a shapeshifting alien supersoldier with Weapons of Mass Destruction grafted onto his body. He doesn't start fights, but if you try to start one with him, he'll verbally annihilate you first (or try, it's sometimes hit and miss, he is 13 years old after all), if you won't back down, he'll disarm you. But if you're the kind of belligerent who is a thug, he will leave you lying on the ground, suffering from pain that will hospitalize you for a few days.
So…How do I write his dialogue so that he comes off as emo, trying too hard to be badass? He doesn't give this impression on purpose.
Source: reddit post