I'd be interested to see some oppositeish takes on paladins. Think Sky-net or ultron, they are tasked and oathsworn to protect humanity, but they see humanity as it's own downfall. Therefore the only way to protect humanity is to destroy it.
Because the Oaths no longer come from deity's the oath really only has to make sense and resonate crazy strongly with the Paladin. No one else. I could definitely see some low int paladins being duped by demons into making pacts "for the greater good" especially if they were disguised and fooled the 'good' paladins into it. IIRC there isn't a specific way to remove a pact from yourself without the pact holders consent...
For barbarians;
I REALLYwish wotc hadn't put so much backstory and fluff requirement into them. Sure you have a lot from back-country brutes. But I'm sure that there are berserkers from all walks of life/societies. In history the franks, who wouldnt have been considered 'barbaric' at the time, definitely had "berserkers", the same goes for the celts. To pidgeon-hole and entire class into the outlander background REALLY annoys me...