I don't think he was nicer. I think he enjoyed his work as a scientist, (I don't doubt that he was still rude) but felt something was missing --like he wasn't content with his position, so when Megatron rose up as leader of the Deceptions, Starscream realized that being a Decepticon (and even becoming leader) was what he had wanted his whole life. It's hard to say since the G1 show didn't reveal much of his previous life