yes, theon has done horrible things, just like 90% of the characters in asoiaf have done. but he is not in any way the black&white villain most of the fans make him out to be. here's a short list of things in theon's defense that people often seem to forget:
- The starks were not his real family and they didn't treat him as a family member, except for robb. he was their prisoner and didn't owe them anything.
- Theon took winterfell in a very clever way, with the least bloodshed possible.
- Ramsay came up with the plan to kill the miller boys, he manipulated Theon into saying yes to it, he did the deed and Roose and Ramsay later acknowledge that Theon was basically their scapegoat for the whole thing. (which btw, makes him still responsible, but there's a big difference between this and "he killed two kids" which is what the fandom usually makes of it)
- Theon suffers from horrible guilt nightmares after he commits his worst crimes and asks himself "How did I come to this?" he clearly never meant for any of it to happen the way it happened.
Theon is not any better or worse then most of the other characters in asoiaf, the ones who haven't commited heinous crimes are often the ones who weren't put in a difficult position where they had to chose between their own death (or worse) and committing said crime.
and before anyone accuses me, I am not excusing the things he did, but there is a reason behind it. he is a realistic person who is flawed and yes selfish (aren't we all?) and he never started off with bad intensions, quite the contrary. please don't simplefy his character.