Ultimate intentions are completely unimportant. No one thinks of themselves as the villain in their own heads. I'm sure Ashur's intentions are good, as are Caesar's, as were Richardson's, as were the Master's yet all of those people were wrong and committing great evil whether they saw it or not.
The difference mostly is that Ashur actually got the Pitt working. A few years back he would have been just another Wasteland upstart trying to found a nation but he actually did it. The Pitt is probably the only faction with heavy industry, I imagine the Pitt could net lots of money by exporting steel and other metals to neighboring societies. Ashur also has a loyal army behind him. Ashur is probably around his 40's so he still has plenty of time to live. He has a very healthy child who will take over after his death. The Pitt is isolated and does not have any real enemies. All is set.
It's very unfortunate that Ashur uses slave labor, but unlike Legion he is putting them to good use. I know I wouldn't want to be a slave but as a 3rd party observer I'd say Ashur is doing everything right. After two centuries of total anarchy, little is truly evil. For all it's worth Ashur shows a lot of remorse for using slave labor, reluctantly declaring "it's just what needs to be done". By the time New Vegas happens, I imagine the Pitt's slaves have already been freed.
Besides, Ashur's motivation is in rebuilding the Pitt, finding a cure to mutations and perhaps found a more centralized, standing army, unlike Caesar who massacres thousands and enslaves the rest just to prove his point. He builds unlike Caesar who just conquers. There is a difference. Like Ashur says it, it's easier to loot a dying city because there isn't a machine that builds it for you.