I can't think of anything that would make John hate someone absolutely, but he could be pushed to the point where he wouldn't be able to forgive them; if someone who he didn't love betrayed him and his friends for a reason that had nothing to do with the greater good - e.g. going and slaughtering all his friends for fun - or if someone was just... plain and pure evil bad guy and they killed someone he loved very much, e.g. what Bec Noir is doing atm. It has to be something really serious, and there can't be any reason he could feel sympathy, empathy or understanding for them.

He'd still be appropriately civil and even helpful to them if they stopped and later regretted their actions, but he'd never quite forgive them.