As former comrades, the brothers knew exactly what kind of person Clown was.
Leonidas: "Heh... If you ask me, he's coming straight for us. Little brother is just his entry point."
Clown had eaten a massive loss in the Silverwood Realm. The ocean clans there had unleashed the Sea of the Dead, ensuring the Cult of Four didn't get a single drop of the spoils. Taking a hit like that, there was no way Clown would just sit back and do nothing. That was the consensus everyone silently shared.
Arthas: "Trouble follows that bastard wherever he goes."
Arthas: "Hmph. The way I see it, this isn't necessarily a bad thing. He thinks he's making a mess for us, but he's actually just exposing his own tracks."
Arthas: "The second he shows his face, we drop a net on him and end it."
Arthas's tone was cold and ruthless. As Orion's mentor, Clown gunning for his protégé meant Clown was gunning for him.
