someone explain this

Yeah, there's no real coherent story or continuity at all. Like they just sit around and say: "wouldn't it be cool if this character did this" and don't bother to set it up, show motivation, consequences, etc. Plus, just like RoP, there's pretty much zero interesting, likeable characters. Not one. The average 8 year old could make better decisions that pretty much every. single. main character. in every situation and yet they're supposed to be successfully running kingdoms and fiefdoms across an entire continent. Pretty much the only intelligent characters are bit players who rarely show up. Daemon is pretty much the only one who has a moment of character, initiative and bravery, but he was such a d!ck before and after that it's pretty much irrelevant. Honestly nothing interesting at all that makes it worth tuning in weekly. We used to watch each GoT episode "live" but with this, we *might* watch is a couple of days later if nothing else is on.

It was stated in the show? Harwin had to leave, as he basically screamed out "YES, I am the father of the kids". Before it was an open secret, but everyone (even his father) was willing to ignore. After the attack and all the witnesses it was impossible to ignore - even the kids finally got it, that their brown hair is not a oddness of nature.
Also Harwin was Leader of the city guard, like the police - so here to serve the people. While Criston is member of the King's Guard, a ultra elite knight troop, directly following the orders of the king.

So Harwin attacked the higher level knight openly and there was absolute no chance on denying what he has done. So this needed to have consequences.
Meanwhile, Criston attacked someone on the dinner feast of the king, likely no one really knows how it started and the witnesses are contradicting themselves. Yes, Criston killed Yoffrey, but he could say that Yoffrey was insulting the king and he overreacted. Also as he had the backing of the queen, he had strong protection. She could have come up with a Hightower witness confirming whatever he said for example. So with reasonable doubt, he might got punished (there was a 10 year gap, he could have served 1 year punishment), but not exhaled. The only thing which was too much was that Laenor was also hit in the brawl. This they should have cut. Also still redeemable for the show (Valyerions dont want to make to much fuzz about the killed lover of the prince consort)