OP - Here's what I think. Take it or leave it.
Your Dad is a hunter. That's who he is. Hunting may very well be all he knows, and what he is proud of. He may not being doing these things to disrespect you, but out of total habit.
So, he leaves gun magazines and gun magazines (two different things here) lying around the house. Bullet shells in ziplocs. (You can reuse the shells - perhaps he collects them and does just that?) Maybe he doesn't just "put" these things out or "keep" them out for the boys to see - maybe they are always there and so much a part of his life and his home that he doesn't even really notice them, or think that they might be a problem. In other words, it may not be done to spite you.
Or, perhaps, he thinks (and rightfully so) that this is his home. He shouldn't have to hide everything when family comes over. I certainly don't make my hubby hide all his gun magazines and gun magazines
when guests come. There's nothing shameful in having them and no reason why he needs to conceal them. If people are uncomfy, they don't need to come back. Same with him carrying a weapon. If people don't like the fact that my DH is wearing his gun in his home when they visit, he shouldn't have to take it off - they just don't need to come back. (I'm not being snotty here).
As for the boys seeing what they saw, if this is all your dad knows - this hunting thing - maybe it was all just done in total innocence. He took the boys for a walk, along the way he checked his traps (and perhaps he wasn't even telling them he was checking them - he may have just glanced at them and moved on without making a big deal of it). Or he may have told them that the traps were there to kill the pests just as people use traps to kill mice and rats in their homes, etc. In other words, it wasn't a big deal to him and it probably didn't even dawn on him that it would be a big deal to you or to them (lots of men just don't think like women do.)
ANyway, then he came across the pitiful gopher. At that moment, perhaps his mind was only on ridding that animal of its suffering, and that's what took over. I know plenty a hunter and farmer who have no qualms killing animals but DO mind seeing a suffering one. He probably didn't think "Oh, here's a dying animal! Let me show the boys how fun and manly it is to beat the life out of one!" I'll bed he was more likely thinking "Oh, good God. The thing is suffering. Boys, this won't be pleasant and I'm sorry but this animal is in agony."
: Multiple beatings with a stick doesn't equal being inhumane or enjoying what you're doing. It only means that the first swing wasn't hard enough or direct enough and now the animal is suffering even MORE, so better get the job done to put the animal out of its pain. If the stick wasn't very big, it isn't unreasonable AT ALL to think it may have taken a few hits. And it's not fair to presume he enjoyed it. As for sending them back to the house while he took care of it, maybe it wasn't safe to do with other traps in the area or who knows what.
I think we should presume the best of a person rather than the worse, esp when we cannot read hearts and know another's intent.