How well do you compose yourself when someone says something negative about your child?

When I went to pick up my stepdaughter from my husband's ex, my stepson wanted to play with my son (my stepson is grown) anyways, I let him get him out of the car, and my husband's ex came out of the house saying "don't you ever bring that kid to my property, how dare you bring that kid around me, and as she was walking in the house she spouted off that with his blonde hair, he doesn't look anything like his father (I have light blonde hair, but he looks just like his dad btw) and ain't even cute" Now my son is 17 months and a living doll, but when I heard her say that, I literally saw red, I have always heard that, but never experienced it. I just ran up to the door and started banging on it, and was really wanting to just beat the crap out of her for saying that about my son. In 8 yrs, she has done some really bad crap, but I always laughed and let it slide, but the thought of her insulting my baby threw me in a level of anger I have honestly never experienced. If I could have got a hold of her, I really would have done something I would have regretted. Why did I get so angry? Is that a parental instinct? Am insane? I don't want to just go off on random people as my son gets older and more involved with people, and look like trash. How do you handle situations like that?