For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some 'organic element' since I couldn't accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
There's quite a few versions of God that have hard determinism built right in, too. It's God's will that everything has a linear course, nothing can be changed, no choices are made, which means people are predestined to be punished for things they literally have no choice in.
In my opinion that's crushingly bleak and believing in and outright praising any sort of divine creator that'd punish their own creations for doing exactly what was planned for them is fucked up on so many levels.
I suppose such an entity would be an interesting premise for very dark horror literature.