"The end always justifies the means."
- — Inquisitorial Proverb
This was a mistake that cost him his life.
Active in the later centuries of the 31st Millennium following the Horus Heresy, he became convinced that all Space Marines were the secret servants of Chaos because of the betrayal of the Chaos Space Marines during the Heresy. He began an all-out holy war against the remaining Loyalist Adeptus Astartes.
Mendoza's action came to a head when he boarded a Chapter cruiser of the newly-formed Black Templars, a Successor Chapter created from the Imperial Fists Legion during the Second Founding. He accused the crew of harbouring daemons and declared them Heretics.
After a swift trial, fifty Space Marines of the Chapter were burnt at the stake. Xavier Mendoza was later found dead, probably assassinated by agents of the Black Templars or the Officio Assassinorum itself.
- Warhammer Monthly 12, pg. 7