I've been pondering Michael's nastiness toward Amber (when he had actually done her wrong) and my theory is...
After the 'whipped' Twitter comment his ego was dented, he then feared a further blow from her possibly meeting someone in Casa Amor, his ego is just too huge to allow the possibility of her 'whipping' him and then possibly rejecting him.
He went for the safer option, a pretty girl by his side, he comes across as the tough desirable man he aspires to be, just in case amber found somebody better.
Only amber proved him wrong and the minute she walked in alone, he regretted his decision and felt like a fool, the anger and and disappointment he felt he then unleashed on her (because fighting with her, making her retaliate would've made him feel better about dumping her)
He is now filled with regret, facing amber in the villa each day as a reminder of what he's thrown away. He'll either try to patch it up with her (by sabotaging his relationship with Joanna) or continue to unleash anger on amber for any little misdemeanour.