This was an attempt by Will to regain his masculinity
He has been mercilessly mocked and had his man card revoked ever since it was made public that he lets other men ball his wife.
You could see a little smirk on his face after the slap like he was thinking 'yeah...i'm the man again'. Really it just made him look even more beta. First of all he slapped him like a bitch. If you're that offended, go for a punch. Secondly, you've gone after someone way smaller than you. Try and attack 50 cent who also disrespected your wife.