It definitely doesn't look like your typical trouser snake, so you can rule that one out.
So, you're an expert on those? Must have seen a lot of 'em...
One that's fixin' ta die.....
I tried to find it, and this was the closest I could find.
Southwestern Center for Herpetological Research - Snakes of the American Southwest - Nerodia erythrogaster transversa
While their initial instinct is to flee when disturbed, they do typically have a nasty disposition. They do not often hesitate to strike or bite if handled,
I understand that snakes invoke a primal fear, but killing things needlessly always bother me. Unless you have small children or know it is a danger (even then I am comfortable trying to relocate. Kind of a rush getting a rattlesnake into a pillow case for the long toss into the marsh), why kill it? They actually are great at pest control. I think you would be surprised how beneficial they are, and little of a danger.