There is a large Shi'a minority in Saudi Arabia in the NE, and to the Sunnis, they are apostates (and vice versa). It was a Shi'a mosque. Think Catholic and Protestants in the 1300's in France and add a few barrels of extra volatility. The two sides have 1,300 years of hostility going.
Well, 16th century. But what’s a few centuries between friends?
The Sunni-Shia conflict isn’t more intense than the Catholic/Protestant one. I’m not sure it’s possible for a conflict to be more intense.
What is obvious is that it’s gone on a LOT longer. The religious wars in Europe lasted less than 125 years, with some aftershocks.