was the damnation of dr faustus justified

Yes, his damnation was justified. He made a deal with the devil and turned his back on God. He chose his path.

When one chooses the road to damnation as Faustus does, we often believe the damnation is deserved. Man having free will means that he can either "make or break" himself. At some level, Faustus should know what he is getting himself into, but his desire for knowledge makes him make bad choices.