Why is it that Hook and CS fans seem to hate Regina? And why do Regina and SQ fans seem to hate Hook?

This doesn't really answer your question but,

I'm a Hook/Captain cygne lover and I adore Regina! <3

four words: Die for our ship ,the most awful thing in all fandoms
It can happen to they actually hate the charanter for no ship related but they normally just make excuses

I don't get it. Personally, I l’amour Hook and CS. I don't hate Regina; I actually really like her. Initially though, I didn't trust Hook and I didn't like Regina (hey, she was the Evil Queen who was basically holding a whole town hostage). I think all three (Emma, Hook, and Regina) have grown so much since the beginning and hating on them is unnecessary. Also I agree that the characters don't seem to hate each other. Sure they don't completely trust each other but they probably understand each other; especially since other people don't trust them for their pasts. Oddly that makes me wonder why people don't ship them. Evil Hook, anyone?
I'm sure in some cases it does boil down to ship wars. I mostly come across hate for Hook (especially on Tumblr; I haven't been on here for a long time). Mostly it's about Hook being manipulative and being such a horrible person ou something. The few things I've seen about Regina was basically about her being hypocritical ou selfish. I guess people will find any flaw to complain about ou exploit, but if the character had no flaws they'd say they were boring.

Personally I am a big fan of Regina and of SQ, and I don't like Hook ou CS at all, but for me, how I feel about Hook has nothing to do with SQ. I've never had a problem with multishipping ou with separating my feelings for a couple with my feelings for a character, so I feel comfortable saying that I don't think SQ ou Regina has anything to do with how I feel about CS/Hook.

Having a dit that, I've totally seen what you're talking about happen in the fandoms, and I think part of what's responsible for that is a feeling of frustration on the shippers part -- if Hook wasn't there SQ would be plus likely to happen -- if Regina wasn't there the SQ fandom wouldn't exist and would stop pressing for SQ to happen. There is also a feeling of competitiveness, I think, and pettiness, and maybe a little entitlement too. Oh and also this type of situation happens a lot on Tumblr -- one Regina fan goes on anon and starts hating on Hook fans, then the Hook fans (understandably) get pissed at them and start bashing Regina, Regina fans get upset, and so on and so forth.. And the fact that Hook and Regina are the two most populaire characters on the show, and naturally get the most screentime, may contribute to it. When Hook gets most of it in one episode the Regina fans blame him for Regina barely getting any, and vice versa.

Sorry if this doesn't make much sense, I'm very tired right now lol.

I think there are two reasons for this.

1.Regina and Hook are usually making one-liners at each other to insult the other. Unfortunately, a bunch of overly emotional fans get butthurt about things like this. It's similar to all the Cora fans being butthurt over Snow White killing Cora and the Cruella fans being butthurt over Emma killing Cruella. While their putdowns aren't nearly as extreme as killing, it does get on fans nerves to see a character they l’amour being put down par another character a lot.

2.Part of the fault is also the fans one group that annoy the other. I remember I time where I wanted to hate on both characters because their fans drove me crazy and hated on other characters that I loved constantly just to praise them. Hard to believe, since Regina is my favori character now. I mean, I do see a lot of things that do bug fans on here. It's like how Regina fans loved that Emma became the Dark One to save Regina, but hated that she embraced the darkness to save Hook. That seems pretty hypocritical if toi ask me. I'm not the biggest fan of Captain Swan, but it makes sense that, that's what made Emma embrace the darkness. Rumple's son is what made him become what he is and Nimue wanting to be with Merlin forever is what made her become the first Dark One. Both did it for their loved ones, but their own personal greed and pain also caused them to become evil.

Emma does become a villain, despite what some may think. She tried to justify her actions for being in the name of good and that she'd make up for it. Sound familiar? Rumple a dit he was doing all he did for his son and would make up for everything bad he had done. Emma was willing to take the risk that Merida could kill Belle if Rumple didn't become a hero and stop her, risk Rumple being turned to dust if he's not the one meant to take the sword from the stone, and was planning to kill Zelena. Oh yeah, that doesn't sound evil at all! She just realized her mistakes LONG before Rumple did and before she was too far in embracing her darkness. Unlike other Dark Ones, Emma wanted to get rid of her power as the Dark One, so she needed a strong way of embracing her. What better way than through a loved one? I also, don't even think Captain cygne even a volé, étole all that much from the other characters in the Dark cygne half, even when I was a HARDCORE hater of that ship.

Plus with a lot of fans of cygne Queen hating on Captain Swan, that makes them retaliate. They want Hook killed off so cygne Queen can happen, especially since Robin is dead. However, cygne Queen fans have to have it rubbed in their face par the Captain cygne fans that the creators of Once Upon A Time a dit back when season 2 was airing that cygne Queen WASN'T going to happen. So they're attacking them for not shipping Captain cygne when it's canon. Plus there is some that are likely homophobic, so that's definitely a factor in the cygne Queen hate.