ppl are forgetting that the curse mark is what started all of this....orochimaru mentioned that it was gonna make him crave power, by giving him a taste of the power the curse mark allowed him to access, oro changed his whole perspective of gaining power, so sasuke thought....since this curse mark actually works...i might as well use it, and so he did
Ya you got a point, but I want to clarify that the curse mark itself did not make him crave power, rather his realization that his current progression would not be sufficient to kill Itachi is what spawned that need for power.
I'm pretty sure that after seeing the power of Naruto's rasengan, how it demolished the water tower in their fight in Konoha, that that is the moment he realized he wasn't getting any stronger. Then he was encountered and convinced by the sound 4 to go to Oro. I don't think he actually had an intention to join Oro before those 4 came for him...did he? If someone can show me manga chapters (I'm going to bed so I'm not gonna check) that prove he had the intention before meeting them, that would be nice. But I don't think he had any intention, via cursemark or whatever, to seek out Oro until he was basically forced to go by the sound 4.
Then after they convinced him to go and after he felt the power of the level 2 curse-mark (spawned on him after breaking free from the incantation cylinder or whatever we call it) he decided to seek out Oro to get even more powerful. Does that make sense? Again, I truly think it was Itachi that was the initial person to corrupt him, Madara the final person, and Oro was simply a median for Sasuke to grow stronger through. Of course Itachi built in multiple paths of redemption for Sasuke to come back from, while Madara has no such intent.