This heavy, so good! Even the plot is heavy, so they match well. I'll try to give my interpretation too: it really feels like a Dr. Jekyll thing - the last track could actually be the beginning of the new cycle. The character cooled down thanks to some pills or acids, the only thing that helps them control their rage. But that's what actually turns the character into a furious monster, maybe due to some side effect, like hallucinations.
That was just my trip, anyway, amazing OST!