Symbolism, Imagery, Allegory
Did you notice there's an awful lot of blood being spilled throughout the novel, all in the name of justice? In fact, so many people die over the course of the novel that we become almost numb to all the bloodshed by the end.
The Op uses violence to destroy violence: he has the gangsters spill each other's blood, but does the Op succeed in keeping his own hands clean? Does the Op try to avoid his own moral complicity in all the bloodshed by thinking that violence was a "necessary evil" to clean up Personville?
Blood Simple
This question acts as a central theme for the novel. After all, what's the point of staying atop your high horse when it's so easy to go "blood simple" like the locals? The Op finds himself tempted by this phrase, and we can understand why. He feels the easiest way to clean up Personville might just be to purge it of its evils (and evil people) entirely—and going blood simple is the fastest way to do that for sure.