"It's been breathing fucking heavy for a long fucking time now and it's lost its fucking will to survive." - Basically seems like the relationship was struggling, which could be WHY he cheated, and by doing so, was the nail in the coffin of the relationship.
"And what's so sick, so wrong that you don't want to believe that such a massacre is capable of coming from me? You're gonna lose your lunch. It's gonna blow your mind." - Obviously about coming clean about what he's done, and betraying the trust the significant other had in him.
All the lyrics after this point strongly suggest that he doesn't necessarily regret what he did, because it made him feel better, and will continue to not regret it (stomping the heart out of his conscience, hoping that it dies) because she keeps giving him shit about it.
Cheating makes total sense.
"It's been breathing fucking heavy for a long fucking time now and it's lost its fucking will to survive." - Basically seems like the relationship was struggling, which could be WHY he cheated, and by doing so, was the nail in the coffin of the relationship.
"And what's so sick, so wrong that you don't want to believe that such a massacre is capable of coming from me? You're gonna lose your lunch. It's gonna blow your mind." - Obviously about coming clean about what he's done, and betraying the trust the significant other had in him.
All the lyrics after this point strongly suggest that he doesn't necessarily regret what he did, because it made him feel better, and will continue to not regret it (stomping the heart out of his conscience, hoping that it dies) because she keeps giving him shit about it.