Trump is not out to score points with those who already support him. He is not out to dissuade those “never-Trumpers” who will not support him no matter what.
Trump is targeting the undecided, and he proved during that first debate that he can address Hillary as “Madame Secretary” while she addressed him as “crazy, insane, sexist” etc.
Trump acted presidential, while Hillary with her botox induced smile looked like a crazy name calling loonie.
The last thing Trump needs to do is to lose his cool and start calling her names. He needs to show that he is unflappable, and he did.
It was Hillary who went nasty in the debate. Her digs were personal and intended to hurt. People saw that.