I'm not into scat, but here's my attempt at explaining.

As with most fetishes, I think it's the result of mostly the environment one grew in or the phenotype, but that doesn't always explain it. I think it's a subconscious process, that of settling what sexually turns you on, and it may not be obvious. I don't think people get to choose their fetishes.

Take my example, I was very socially awkward as a child and didn't interact much with girls, but then again so are many and not many end up liking guro. My parents were over-protective and I wasn't really mistreated in any way, but that made me into an introvert. (now I'm kinda in between, as I've realized that social contact is critical to a successful life). I really can't tell why I get off to the thought of slicing damsels throats while fucking them. Another weird thing that differentiates people even more is that there are plenty of people who are into guro/snuff but also feel empathy and wouldn't do anything to harm our partners irl (me included). Haven't really done any research on the matter, but perhaps genotype also plays a role and who knows, maybe through mindfulness and repeated introspection one can get to the bottom of why they like something and willingly change what they like and not.