Michael Collins was a general thorne in the side of the British and of course of De Valera. He was a very smart game player behind the scenes, but he was not strictly a politician.
There are several trains of thought on who had him killed. Some thing it was the British, but the other train of thought is that Eamon De Valera had a hand in it, as he was more popular with the Irish than he was.
When De Valera was asked about Collins's death and his involvement he is quoted as saying "I can't say a thing John but - that fellow had it coming to him."