greyjoys: disgusting vile insects

this house always particularly confused me. like somebody explain this to me plainly.

1. they weren’t ruled by the 7 kingdoms or a part of westeros correct? or were they?
2. how did Theon end up w the Starks? I’m just confused on what, when, why & how?? because they seemed pretty sweet on each other in the first episode but then when Theon eventually turned into a Tyrwbr & killed the people who helped raise him he said he hated the starks?
3. and why so fucking evil? Theon & his sister ended up coming to fight for Dany & ik so they could get their home back but why take Winterfell & shit before this?

Theon just seemed pretty fucking grown whenever the shit started so if he was their prisoner he could’ve just left whenever he wanted even before the war & Robb went off to war