Mescaline is a hallucinogen. If you consume it, you hallucinate. When you hallucinate, you see things that don't really exist. Parallels have sometimes been drawn between seeing things that don't exist and being crazy.
If one is intelligent enough, one can take mescaline and if you're lucky have a wholly entertaining experience that does not make one turn into a bible thumping idiot. If one is too foolish, one is doomed to be reduced to idiot status, if one is not already there.
Mescaline can give good experiences or horrifying experiences, it can make users sick because it increases the heart rate and lowers blood sugar, moreover serious overdoses can be fatal. Under the drug's influence users can do crazy and insane things that they may later regret.