Will did not have a real-life, loving, great childhood like most of us, so I have to overlook the whole "he shouldn't have embarrassed his mom" thing. Sami used him as a pawn for most of his childhood, ruining his family life, and walked out on him during his most vulnerable moment because she could only think about herself.
And while she tried to help him with Nick, it usually only made things worse (such as Gabi's panic attack when she got pregnant and went to the hospital, and when Gabi almost took Arianna away) and Will always told her not to interfere. Sami doesn't listen, she only cares about herself and being right. Will gave her a case of "tough love" that is much needed.
Sami is a judgmental hypocrite and an awful person. Will never said that Abby was in the right, innocent or any of those things, so I don't get why Sami and many people here keep insisting on that. He DID tell Sami not to go any further because she already did enough to Abigail and would only hurt herself and the kids in her revenge plot.
Will he regret writing it? Maybe a bit, especially once Abby's name comes out: that was not his intention. Bt when he learns Sami was behind it and threw him under the bus, he will think he was justified in the end.