Why can't it both? Logic: Cause and effect.

His extreme sexism came after his warped mind became evident.

There are lots of sexist people (both against men and women) who don't become genocidal murderers as a result. Not that this justifies sexism, but as someone who has a mental disorder, I would appreciate this chance to learn more about something that people are painfully unaware of the extent of, to, you know, learn about mental disorders, and what they can do to people when they aren't properly helped through it. Not have it get hijacked by another "because sexism" piece of drivel.

Sexism is bad, but it didn't cause this mentally unstable person to kill. Being mentally unstable did. Stop detracting from the real problem in this case with a different one that was a symptom, not a cause.