He is the demon. Every relationship, friendship, ect. that's he's started either drifted apart or he ruined. "I'm the one who ends up leaving," as in he would be the one to end it & any problems with it to "make it okay." At this point that he's "broken" it's easy to see through and understand him. "Jet black heart... and a hurricane underneath it, trying to keep us apart," means that he's just naturally a bad person but when he really wants to be with someone, even he can't control himself. The "poison pen" means that everything he does or "writes" is poison. "The chemicals moving between us, are the reason to start again." Whatever is pushing them apart can't be reversed, so they're better off just going back to the beginning. "Holding on for dear life...nothing that could make you stay." Again, he's trying so hard to make the relationship work, but still even he can't control it. "Blood in my veins... made up of mistakes... forget who we are... dive into the dark... burst into color." He just wants them to forget all the bad things he's done and just start over and turn into something new. Basically, he's always been a bad person with a bad reputation but when he has this one relationship, he wants to make it work but because of his past, even he can't make it work regardless of how hard he tries. So, he wants to just start over since there is no possibility of rewinding. Hope this was helpful! :)