I didn't see MJ as being mean to David. She was a woman who was finally done with him and his passive aggressive reactions. They consistently hurt each other, broke up and made up in vicious cycle for years simply because neither was willing to let go completely. With the betrayal, MJ finally felt done and took the steps she hadn't been able to take before.
He played a large part of the wedge between MJ and Lisa. If you listen closely to his exchange with MJ, he was quick to say "It's not your fault" with little to no acknowledgement of his role in this mess. Yes, I get that he was hurting, but he was wrong to squarely put this on MJ.
I'm glad this was a strong episode that addressed tough issues such as depression, mental illness, suicide and toxic relationships. Lisa was beautiful, accomplished and tortured. There are so many women like her and I'm glad it was addressed in a realistic way, not for laughs or with a quick 5 minute wrap up.