It's not that you're in the wrong by protecting what's yours. It's just that your approach could use some work. Basically, what I'm (and I think others here) am saying is that you could have handled it in a more diplomatic manner. A simple 'could you please stop using my business logo as your avatar' in a PM would have sufficed, but instead you took an extremely aggressive and public approach, hunted down (from what I saw) every post he made and replied to it with said aggressive post, and then were surprised when he (eventually) got defensive.
Copyright infringement isn't ok. However, this is the internet. If he saw an image he liked, no one had claimed it was theirs/copyrighted/whatever, and he honestly didn't know who it belonged to, there was no notice not to use...how was he supposed to know? Just for the record theft isn't the same as copyright infringement.
Long story short, you can't blame him for how you handled the situation, and imo your approach largely contributed to the problem. You are in control of your actions and your approach to situations. He apologized and immediately changed his avatar. You continued your highly aggressive approach, and he (predictably) got defensive. That's human nature, especially at his age.
To be honest, this entire fiasco is somewhat appalling to me, as a new member of the forum. I really hope it isn't typical.