It's been several days, he had plenty of time to wipe stuff down and dump it. I don't think this is arrogance necessarily, perhaps isolation and paranoia.

Probably hadn't really thought through the specifics of tossing evidence.

Bodies of water are popular for disposing of things, because it's a huge PITA to find things there. Imagine how hard it might be in a large local pond, and then multiply that in complexity for a rather big river. Or an inland sea like the Great Lakes. Don't even multiply for those; it's a stupidly big irrelevant number.

It's hard to find cars 20 ft from the shore and a huge number of missing persons disappear that way accidentally driving in.

> I doubt he was arrogant. I think he was on the run and didn’t have any place to put away the evidence. Probably homeless this whole time. And had to go to get food at some point.

He totally did. Find a trash can, and put it in (though maybe disassemble the gun first, and dispose of it in pieces in multiple locations).