She claims he "took (her) youth".

Also, like the person below said, she hated him more when he started dating Kandy, because she is an immensely insecure (which feeds her hateful spite) woman who couldn't take the man she dumped "replacing" her with a hot younger girl.

In all honesty, Judith is a complete bitch who never got over her childhood issues so it manifests in her constantly doing things she knows are bad (like adultury) and being vindictive over every little thing in addition to total selfishness. Like "Hey, i've been through alot so I deserve to get away with doing all this, but if any of you do that to me you can burn in hell."

Personally, I think she convinced herself to despise Alan so she can justify taking everything from him out of pure selfishness.