Theon was never a good guy. He's just now turning into one. The iron born are like the white trash of Westeros
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You're missing the point of Theon's arc. "What is dead may never die but rises again, harder, and stronger." This quote encompasses and represents Theon. Theon was "dead" and replaced by Reek. Theon is now making a return and is going to come back on a path of redemption, better and stronger.
To say he was never good is false. He isn't evil and didn't want to hurt anybody when he took over winterfell. He assumed there wouldn't be a struggle from anyone. He wanted to be respected. He wanted to make a name for himself. He wanted his biological father's approval. He had to do these things or risked being mocked further by his men.
He loved the Starks. He didn't want to kill ser Rodrik and he killed the farmer boys to protect bran and rickon. He has carried remorse and guilt for these things ever since he did them.
To say he is just becoming a good character is wrong. As most characters in this series he is a gray character. As I stated earlier his arc represents redemption. Start high, drop to rock bottom, come back stronger and better.
He loves the Starks, they are the only "real" family he has ever had and he shown that ever since Sansa ended up at winterfell and prior to that when he saved bran and rickon by killing the farm boys.
I didn't mean to rant, I love this series and the books more than any series I have ever read.