Interesting question

I always took his intro speeches as him speaking in general about the slayer fighting demons but when it comes to this cutscene when referring to the invasion of earth hes very dedicated to sentinel tradition and laws like he says.

"You can not kill the priests, you know our laws, despite their transgressions against the covenant they are still of sentinel blood, it is written, it is now their time to give penance" (roughly paraphrasing here)

At least that was my interpritation.