I would think the mothballs would be toxic I've never used them myself not even in storing things in boxes I have used cedar for that, I just don't like the smell of mothballs really,
I have a set of snake tongs that I use to grab the snakes once I get the trap open, with the copperheads I'm just extra careful opening the trap, I suppose that shooting the snake in the trap would be okay might damage the wire some though, there have been several good suggestions about how to get them out, shooting, freezing, drowning ect.. I would think freezing would be the most humane if you weren't going to relocate them. For the poisonous snakes I freeze them after I take them out of the trap I don't relocate them, I'm not making my problem someone else's problem.
I have caught several more snakes since I first posted I'm at 9 for copperheads 3 pigmy rattlers a couple more rat snakes one king snake and a garter snake LOL not sure what it was doing hanging out around the chickens nothing there for it to eat. I have lost count on the total snakes somewhere around 15 so far the trap does seem to do it's job well,