Sam claimed that being soulless had a few benefits that allowed him to become a better hunter: He didn't need to sleep or to really eat anymore, he had a higher pain tolerance and was able to lie to the goddess Veritas, even though she could make all other people, even Dean, tell the truth against their will.

When did Sam become soulless?

After his soul was reunited with him, Sam again felt great remorse for his actions and spent the rest of the season struggling to make up for what his soulless self had done. In Season 7, after Castiel brought down the wall in Sam's head, Sam began to lose his sanity as a result of the damage done to his soul in Hell.

Is Soulless Sam evil?

Evil! ... In Season 6, soulless Sam seemed a bit evil, due to his lack of empathy, but was generally referred to as RoboSam. At the beginning of 15.04 Atomic Monsters, Sam has a nightmare where he embraced the demon blood and became the leader of the demon army as Azazel had intended.