Part of the reason he's left parliament is that he's unhappy with some of the ways that Theresa May has started to move government policy to the right. He didn't want to support the new policies but he didn't want to make himself unpopular with his party by opposing them and being accused of being disloyal and a spoiler. So he left early.
He left because (a) he didn't want to spend the rest of his political career being blamed for breaking his country, and (b) because the Foreign Affairs Committee was about to release a report highly critical of his intervention in Libya.