Your parents are twats, just sayin'.
Also, you definately had waaaay too high expectations. Especially since Araya can't even headbang anymore.
To be honest, I was a little underwhelmed when I saw them on the Mayhem fest last year, but for me there were a few reasons that came into play;
1. I was tired/thirsty (After hours upon hours of waiting with Trivium, Cannibal Corpse, and Killswitch Engage playing in a row before Slayer's set, I think anyone would be fucking dead).
2. Manson fans in the crowd (Do I really need to explain?).
They still played a great set though. Hopefully when I see them next month with hopefully a real thrash crowd (considering it's them with Megadeth and Testament, I'm not so worried) it will be a better show.