As I recall, he was shocked by how he couldn't steamroll Strahd, at which point Strahd just assumed that Soth would fall in line and went to go rest since the sun was rising. Soth responds to that insult by going on a rampage through Castle Ravenloft and slaughtering a number of Strahd's servants before leaving.
He later agrees to work with Strahd after a subsequent escape attempt fails, and Strahd directs him to another possible avenue of escape in Castle Hunadora, Duke Gundar's home. When that one falls through, Soth marches on Castle Ravenloft in a fury, intent on slaying Strahd, who holds him off with wave after wave of men a sacrificial army of conjured undead supplemented by mercenaries, before using Caradoc to send Soth into the mists, knowing that the death knight will throw everything away to punish a treacherous servant.