He doesn't back down when it escalates further though, at least as far as I know. He's not a punk, he's actually got toughness. Just because he doesn't join in throwing the punches doesn't mean he's a coward. Why should he, when the guy who tries to retaliate will get thrown out in all likelihood? If your opponent is acting stupid, don't join them in their stupidity. I have no doubt in my mind that if he got in a situation where fighting somehow was actually the best option, he would do it wholeheartedly.
Nothing wrong with using words to get into people's heads if they're so thin skinned it will actually effect their play. Does he cross the line a lot when he does his trash talk? Absolutely, but with a few exceptions (he said he never called Villanueva is the one coming to mind right now), he's owns his talk and doesn't apologize.
From what I've seen, DeMarcus Cousins doesn't own his actions, but instead just blames the other team or some nonsense. Instead of trying to get in Blake's head, he complains to the media that Blake is "babied". Instead of being the one to make others lose their cool, he's the one that loses his cool. He and Garnett don't share much in common. If anything, he's the sort of player Garnett probably goes after the most when it comes to trash talking.
"I'm a passionate player. If you don't like the way I am ... I play this way the last 15, 16 years. I leave it on the court." Those are Garnett's words, and I respect him for not trying to pretend to be anything else other than who he is.
(Source: http://www.sportingnews.com/nba/feed/2010-11/trash-talking/story/villa nueva-kevin-garnett-called-me-a-cancer-patient)
Say all you want about him being a punk, but I'll take a so-called punk who owns up to who he is and what he says over someone like Paul George, who's so stupidly buddy-buddy that he wants one of his supposed rivals (Lebron) to tutor him.