Although wrong for taking the lives of others, it looks like there was some validity to his claims.
Two separate inspectors agreed with him.
toronto.citynews.ca
I don't think he was mentally ill. His emotions and pride pushed him over the edge.
Two separate inspectors agreed with him.

Documents detail Vaughan shooter's conflict with condo board
Documents obtained by CityNews detail the Vaughan shooter's dispute with his condo board and his belief that they were conspiring against him

I don't think he was mentally ill. His emotions and pride pushed him over the edge.