First and foremost I think most of her issues stem from being sold at the age of thirteen by her own older brother. The boy who was supposed to love and protect her. Sure his heart was in the right place (pretty sure Izzy herself acknowledged this) but it doesn't change the fact that she was in fact sold to an abuser. And when she confronted Guy about it, all she wanted was an apology. But she didn't even get that...in fact he blamed her telling her that she didn't make the most of the chance she was given. She tried then to explain the obvious, that you really can't make the most of someone who is abusing you. In the end Guy waved her off, calling her foolish or something of the nature. So not only do we have an unsympathetic and remorseful brother, we have an abuser.
Yup, for anyone who managed to ignore it, Isabella was abused. And she was abused in every context of the word; physically, mentally/verbally, and sexually. We saw Thornton smack her, we saw how roughly he grabbed her chin, we saw him force her to dig him up some treasure, and we saw him try to kill her (strangle her to death if I remember right). And those are just the physical torments we saw him bestow upon her. In light of the verbal/mental abuse we've seen Thornton screaming about how he'd kill her if he found her (after she ran off in the digging scene). She was so terrified of him she tired to run away...hell, that was the first thing we saw her doing...it was her introduction! In said intro, when she was captured she begged the guards to let her go saying "no you don't know what he'll do to me." And it was very well implied that he had sexually abused her as a young girl. In the same scene where he tried to strangle her he informed her that the more she runs away the worse she makes it on herself. The only reason he didn't proceed with murdering her was because she promised to 'be a good wife'. Right after he forced a kiss on her. I highly doubt that that was the first time he forced himself upon her, and I highly doubt he held back with 13 year old Izzy. So by all means, her apparent distrust of men has some pretty solid roots. Again, not saying her view point was right...just understandable.
So then we turn to something else that could solidify her reasons for turning evil. I think it's fair to say that Robin led her on and used her. He had her give him her expensive jewelry and he had her help him break into the castle among other things. He would kiss her and he'd take her out on little dates, in one episode giving her a bunch of strawberries. And then comes the drowning scene. As they were about to die she confessed to him that she wanted to run away, find a nice farm house, and have children. He even played into it saying 'and she'd have your brains.' Right when they got out of that situation, right after she poured her heart out to him, he told her that it would all be just a dream. He told her it could never happen because he had to do his thieving thing. That's understandable, however if he knew he couldn't make a life with her, he shouldn't have been fooling around with her. Basically he only fantasized with her because he thought it was the end and he never really had any intentions of going anywhere with their relationship. On top of that, he probably wasn't even over Marian at the time...in which case he probably should have held off on dating anyone. Isabella ultimately went dark because she felt used. She was hurt again.
Even the time period she lived in was a factor. Women in the Robin Hood era were certainly not treated as equals. And Izzy, she's a very good example of this. The very fact that she was sold for a profit to an abuser is proof of this. Further more she was pretty much sold again; (now I don't quite remember what happened but I do know that she was arrested by Prince John) another man (I don't remember his name) came by and bought her. He claimed he could keep her under control. Sure she was out of prison but she was with this dude bro who didn't really have a twinge of respect for her. Said dude bro dragged her off to Prince John's coronation. And then came Robin's coronation cockblock scene. Prince John hid behind Izzy. He used her as a shield. I will give leeway in the light that she kind of volunteered it. But that wasn't even the first time she was used as a shield...Guy did the same thing in Let The Games Commence. Again order may be mixed, but I think it was at this point that Prince John decided to make her sheriff.
Which leads to my next point. Isabella was just trying to survive. At this point Izzy has been so mistreated and what not. She just wanted some control over her life. In Prince John she saw an opportunity to get herself some political power...some stability and safety. She was one of those people who did anything in their power to get to the top so to speak. Yes it did require lying and manipulation, but it was to survive. And when she got the sheriff position it did start rather smoothly. The first thing she did was save Meg; a girl whose abusive husband was going to have her executed for some trivial thing. Isabella freed Meg declaring that she was going to try to make things fairer for the women. This of course got all screwed when Thornton made his big comeback. That's when she started becoming a corrupt sheriff.
I think that's all I have for now. Isabella had gone through so much abuse from her own brother and Thornton. She was used and mistreated. While it doesn't make her deeds okay, it certainly does add some sympathy and multi-dimension.
