In the Comics he goes on to explain that he could not kill Mark(as his time on earth changed him) and fled Earth.

Then as a logical conclusion, because the viltrumites will eventually come find Nolan and also Earth he decided to conquer another planet as compensation for not conquering Earth. But then as he spends some time with them, he realized it wasnt just because of Mark that he couldnt conquer earth but also because Earth had changed Nolan. Realizing this he understand why he is attached to the thraxxans and then goes on to protect them. So here Nolan made an honest mistake by assuming he can successfully conquer other planets because is viltrumite, but later on to realize he is now also somewhat human and eventually failed to conquer Thraxxa.

In the last episode tho he just looks like a selfish asshole who just let the Thraxans die because he was careless and kinda selfish, even tho he was broken. What do you guys think?