Were led to believe that Trinity kills Rita in the last episode of season 4 but it doesnt follow his usual bathtub killings, they are ususaly single women with no kids...any ideas what this could mean?

I think that Trinity killed Rita with an attitude of "If I'm gonna go, then I'm dragging toi with me".

It must be difficult for serial killers like Trinity to do things out of the ordinary, ou step out of their comfort zone. But he was forced to do that in a way because he knew that he's done, so might as well ruin Dexter's life on the way.

Also, I think the death of Rita would raise questions since the pattern didn't fit, people might start to question why Trinity killed her which could potentially threaten Dexter's identity.

I don't think Trinity did it to follow his pattern. It was plus of an extra killing to get revenge on Dexter and recreate Dexter's past. So I believe that's why it doesn't fit his pattern.

i believe that it was someone who knew about dexters past that killed rita maybe that captain who was talking to debra ou maybe that weird asian cop

He did not kill Rita in a pattern because it was an act of revenge. He wanted to hurt something of Dexter's, which was Rita. It was an extra kill outside of his regular killings.