Agree with SelfLoathing. Also agree that your anger OP is completely and utterly natural.
I have known many, many OW. Some were plain nasty. Some had been lied to through the hilt, with husbands saying their wives were abusive, mentally ill, in the process of divorcing them, leaving them for another man, dead (in one case).
I have personal experience of this. My Dad left my mum for her best friend. I hate her because of this and always will, particularly because IME her behaviour is worse as she was meant to be loyal to my mum. Also my Dad was emotionally abusive to my Mum and she knew that and so I feel this was even more of a betrayal on her part.
However, it's not always black and white and unfortunately the onus is on your husband to stay loyal to you and not to leave you for OW. Unfortunately the sad fact is if it hadn't been her it would have been someone else. That is totally true in the case of my Dad too.
90% of the time I guarantee the OW has been fed a load of utter bullshit by the husband. They will believe what they want to. This doesn't necessarily make them evil people. Some are, naturally, but it does very much depend on many, many factors.
IME, you will waste away your life trying to work out 'why' another woman would do that, when the answer could be any number of things.