I'm very sorry you're going through this. However you're feeling, you have a right to feel. When someone commits an offense against us, we're going to have a lot of different emotional reactions to work through and that's normal and healthy. You can recognize that your friend has a problem and didn't commit this act out of malice, but that doesn't mean you have to be okay with it. If you feel betrayed, angry, etc, that's all right. You don't owe him understanding when you're hurting. Self-care should be your primary focus.

A little over five years ago I lost my home to a fire. The fire itself was deemed an accident, but because of the incompetence and neglect of the fire department the house burned down despite being only one block over from the fire house. I've had years to deal with my feelings about this and move on, but it still hurts. If I knew a friend had started the fire--even a very sick friend--I imagine I would have felt betrayed and angry and deeply hurt. I don't think that would be at all unreasonable.

Maybe put distance between you and this friend for a while, to let yourself process. I'd definitely recommend some mindfulness practices. Be aware of how you're feeling, feel it fully, but don't let it consume you. Good luck. If you need to talk to another survivor of a house fire, I'm here.