Maybe there is some subconscious sense of anger towards Joseph for seeming so... okay with his position. He sees Joseph and others like him as cowards and victims, though he doesn't really intend to. After all, he escaped his own slavery when he was only thirteen, so why can't they? Apart from that, Joseph could free him, probably without being severely punished, if Alexi is as "reformed" as they say. So yeah, some of that might be playing into it.
But largely it's just that he's (understandably) in a pretty bad mood. It wasn't intended for him to really be disrespecting Joseph. It's more like, he's angry at the situation, and he's partially angry at Joseph for being a part of it, even though he sees Joseph's part as forced. Apart from the anger, he's ill, in a lot of pain, and he's feels humiliated by all of this. And well, he's not the most polite person to begin with, and he is accustomed to giving orders to those under his command. He certainly doesn't see them as slaves, but it's a sense of trying to get things done efficiently.
But yes, I do see how it came out that way. Although I'm actually intending for him to become considerably worse towards Joseph when he loses his mind, along with Alexi.