Re: Play the game
So he'd been a good employee for years, then spent a few hours, spread over the course of months on a personal project and you'd sack him?
Well done, you just lost years of experience and a good worker for no good reason (and are going to have to pay recruitment and training costs for his replacement). Had you instead talked to him to give him a warning you'd have kept that experience and skillset, avoided the replacement costs, and he'd probably not have repeated the behaviour once he knew how seriously you viewed it.
I'll never understand the mindset of those who think firing should be the primary course of action following a mistake. You might feel right and just doing it, but 99% of the time what you're actually doing is hurting the business.