Tommy Wiseau was fed up with this world. He did NOT hit Lisa even though she said he did. This was the world according to Tommy before the drugs. Tommy later talked with Denny's drug dealer to get some cocaine. His psychologist Peter would advise him not to if he were a real doctor and not some guy off Craigslist.
The drugs Tommy had took that morning started to kick in. His mind was for the first time truly clear and he began to see the world in a new light. It was not Lisa that was tearing him apart ... it was God!
The drugs did more than give him a high, it removed the shades of lies from Tommy's eyes. His dead end job at the San Fransisco bank and 6/10 fiancée at best did not satisfy him as he once thought. His reflection in the mirror frightened him as he now sees how hideous he looks. His best friend Mark wants to do nothing but toss around a football. Lisa cheating on him was the least of his worries, this is why he committed suicide.