He didn't kill himself
He didn't abuse prescription medication. He had an allergic reaction to the sleeping medication which caused the antibiotics that were fighting pneumonia to shut down his body. Or his body was fighting a small percentage of cancer. Which would mean that his body was depressed.
I have this idea for a movie. A world in which people are allergic to the atmosphere. Without taking a pill the person allergic is comatose and slowly drift away. The longer they stay comatose the longer they drift in a house of leaves. They take the pill and they're alive again. The devil is going extinct and he needs lost souls to survive.
We have what we seek, it is there all the time, and if we give it time, it will make itself known.