How do I "move on" and interact with him? What should I do?
One simple word... Forgive
When I was 12 or 13 I was outside with my older brother and we were playing around with a new BB gun we had. There was a pigeon on a wire 60 feet away and for the **** of it we tried to shoot it down.
As you point out, it's not as if the rest of us are without sin. Try to view his actions aginst this cat as his "pigeon" event.
They tried to tell me that he understood what he did was wrong end he regrets it now, but based in the way he was laughing about it, I just don't buy it.
See, he has already had something of a change of heart.
It will also help if you try to see things a little bit from his perspective:
He actually told me that its no different then throwing away a mouse In a trap.
Try to see that there is a grain of truth to this statement. What is the difference between a rose and a weed? "Desire". At its simplest, the definision of a weed is "an undesirable plant". What is the difference between a "pet" and "vermin"? I would argue the same thing... desire. After all, some people keep mice as pets, some see them as vermin that need to be eliminated. So it makes some sense to me that there will be people that see all cats as "pets", while others would see ferral cats as "vermin". We as cat lovers obviously find it impossible to consider cats, even ferral cats, as vermin. But we have to see that not everyone sees cats as we do.
Ultimately I think it's something I'll have to internalize and learn to live with since I'm helpless and powerless to make him change.
You won't be able to turn him into a pet lover, but I get the feeling that because of your willingness to confront the situation, you have already caused change. It seems to me that you keep focusing on the wrong that was done to one cat. As others have said, no mater what anyone does, what has been done can not be undone. But think of what difference you might have already made to the lives of the other ferral cats. Sure, your BIL isn't going to suddenly begin to love and care for them. But at the very least, I doubt any of them will face getting locked in a cage and getting thrown away to die.
In other words, I think you've already made a difference. Try to hold on to that.