A lot of propaganda aimed at conservatives paints liberals as bad, so it is natural to assume that the reverse may be true. Also, liberals often talk in an extremely high-handed manner, see e.g. Bill Maher. Also, the vast majority of Obama's economic stimulus dollars went towards cities rather than farmers. The list goes on and on.
It REALLY didn't help when McCain picked Sarah Palin to be his running mate - she made fun of the liberals ruthlessly, and was made fun of by them in turn, and also the media including comedy shows. So the "stupid conservatives" meme... regardless of having some actual basis in fact, is something that they do not enjoy being portrayed as. I know, I know, best not to dish it out if you cannot take it in return but... either way, there it is, a fact that now needs to be worked around. Conservatives are killing people including women and children... but you can't SAY that, without them getting mad.