I didn't say it excuses it, just that it explains it. Xander's not necessarily aware of his condition or thinks like most people that you should try and have a relationship. If you have these patterns deeply ingrained pretty much the only thing you can do to prevent yourself from hurting your partners is to not date.

Buffy also doesn't have good relationships and for all we know Willow does have loving parents, they're just not shown. Anyway people react to parents with bad marriages differently. Angel was a bastard when he was human and needed a horrible dose of guilt to become what he is; Spike has transferred his devotion to mummy to Drusilla and then later to Buffy, both women who love him less than he loves them. It's different patterns. The only person here who is having good relationships is Willow.

Xander doesn't mean to screw up everything he gets into. He ends up sabotaging these relationships because he's afraid of going for it really long-haul, in case he just ends up hurting them more. There's no evil intent there.