Although it is not right to say that 'left wingers' have more of a right to hate their candidate than conservatives, it is correct to say that Obama has not fulfilled very many of the promises he made to his party. Like any other deceptive candidate, it is right for his party to despise him.
Any bad idea Bush ever had, Obama has been continuing them. Patriot act, continuing the Iraq war, wire tapping and of course the drone strikes. Honestly, anyone that likes Bush cannot say that Obama is 'a far left wing socialist' because he's basically been consistently doing the same things as bush, save for that he got his way up to being a president without the fame and wealth of his daddy.