And there you go again... Mysogyny? Dear goodness...
Sun's carefree and careless atitude aren't motive for him to deserve being treated the way you want him to be.
That time, in a sense, yes, he may have deserved the slap.
You seem to forget that Blake acts like a lone wolf and his presence may actually help her to break the ice and decide to become a better person.
Then she may remember she has a friend in need of her support and return to where Yang is. And the team gets back together, long the way.
If Sun, along the way, becomes more tactful of his surroundings, that's a whole other point RT could look at
