When he arrived I bet him I could beat him
Was the bet about fighting?
No, the bet was about a video game.
Betting is okay as long as it is friendly, but you shouldn't expect to always beat people. There are times when you are going to lose, so you need to be able to accept that as well.
No way. I was going to beat him up, I think he's so weak after all.
Well just make sure to still respect him. Although you may truly believe there is no way that he could beat you, remember that there is always a chance!