I wouldn't describe Diana as a 'villain' but she was manipulative, her relationships with men were characterised by obsessive behaviour, she was a lover of using the Press and certainly not an angel (as she is often characterised). Charles didn't behave well either. I don't think one was better or worse than the other.
I think the relationship with Oliver Hoare, the art dealer, showed her neediness and obsession. She had a relationship with him for a couple of months, he was married. He ended the relationship. What followed were months and months of anonymous phone calls to his house. His wife eventually reported this to the police. They investigated and the calls were traced to Diana's private line in her apartment at Kensington Palace and phone boxes near to the palace. There were thousands of calls. Diana was spoken to by the police and she said it must have been a member of staff. She was told the phone calls had to stop and they did.
A very short relationship with Will Carling, a married Rugby player, resulted in him quickly backing away after she began buying him clothes and expensive presents and the Press being alerted to their meetings- it made him uncomfortable.
I think she was looking for what she never felt/saw as a child- unconditional, secure love. However, I'm not sure she knew what relationships are really about. She equated that kind of love with big gestures, public displays of proof of it and romance, not the nitty-gritty, hard work, sometimes dull, irritating times you have to work through. And it seems like, in friendships and family relationships too, she was a person who was self-destructive, controlling and huffy and would go NC for periods if she felt someone was not doing as she wanted- her brother, one of her sisters, her mother, friends, Sarah Ferguson all suffered this for shorter or longer periods. It's often a sign of insecurity and self-protection.
Charles was unsuited to her neediness. He was an adult, a man whose world met his needs- his work, his very established friends (he still has the same close circle), his projects, his influence, his tastes and pastimes. Her only choice was to fit into his world and she didn't and she couldn't change him. It was never going to work.
His behaviour with Camilla was what POW did - I don't think there has been one before him who didn't behave like that. There are titles in the British nobility created for the children and mistresses of ex-POW and Kings.