Cesare, and the underworld by extension, believes Steve caused the Kellwasser event, a global extinction event, on purpose, and the set up a sytem of Watchers to monitor the planet for other trickster entities so no similar event ever happens again. They've been monitoring Steve under the Earth ever since (hence the line from DOWN "Chaos Gods Rise From The Deep", they believe Steve is a god of chaotic destruction), and make him Cesare's #1 priority when they sense that he's heading back up to the Earth's surface. Cesare genuinely believes Steve is an evil entity bent on wiping out life on the planet for a second time, which is why he doesn't pull his punches when trying to catch him. However, Cesare begins to doubt what he's held as truth for a thousand years when Steve shows a lack of knowledge/understanding of the fact that he's been under the ground for millions of years (if Steve was unconscious all that time, it could have seemed like only a moment that he was down there) and also calling into question whether or not Steve even remembers/intentionally caused the extinction event in the first place. If this is the case we'll just have to wait and see how Cesare reacts to finding out that he's been "living" a lie for the past thousand years. Him saying "Did you not know?" in response to Steve's confusion while in his jail cell makes me think he may possibly have a change of heart. I doubt the Underworld will take Steve's word at face value, obviously, but Cesare seemed convinced, or at least willing to be convinced, for a moment there.