Lets see... if we're talking United conduct only, it might be a tad OTT but his whole career into account, he definitely has been guilty of gross misconduct.
- dishonesty/deception - constantly lying about his own failings, using the fans when it suits his agenda, the way he spoke about Luke Shaw constantly needing coaching and then put on a pantomime show of having to constantly talk him through a game.
- sabotage - The West Ham game alone was example of sabotage - there is no way he picked that team thinking that was the best side and tactics to get a result (in the past at other clubs, he has previous for picking sides with a point to make to the board with no intention of getting the best possible result), his press conferences during pre-season saying fans shouldn't come out to watch the club play.
- discrimination - This doesn't necessarily have to mean racial or sexual (though some might point to the Eva incident) but its clear that he discriminates against certain players and has backed others to the point beyond reason. He picks favorites which any manager is entitled to do, but he then ostracizes players who have tried their best for him just out of spite. He didn't use to do this.
- harassment/bullying/abuse - these can all come under one umbrella.. you just need to look at the treatment of Shaw for over two years and Martial. Relentlessly digging them out publically, criticizing their performances without due reason and taking them in and out of the side, messing with their heads and making out to the fans that these players are 'lazy' and 'entitled'. Both these players went from fan favorites, to becoming pariahs and considering their age - must have been pretty tough to take mentally for them.
For me, the second and fourth points are the most worthy of condemnation and sack-worthy on grounds of his conduct. The dishonesty and discrimination points are clearly not worthy of sacking but taken as whole, they don't paint a pretty picture.