I haven't seen one around here in a long, long time. So I reckon I'd let it go.
On the other hand, we have lots and lots of cottonmouths. I kill them on sight and will continue to do so until I start seeing rats or other vermin.
We've killed them with .22LR, .357 Magnum, .40 S&W, .380 Auto and my son got one with a bow and arrow. I think a short .410 shotgun would be ideal, but they never seem to be around if I have a shotgun.
You don't suppose they ate the rattlesnakes do you?