There is a very real and legitimate reason why, in the current political climate, reasonable people have much greater reason to fear groups far to the political right than the political left. In contemporary American political rhetoric, there are no sizable, nation-wide left wing organizations sincerely advocating violence against either the US government or population. However, there exist certain fringe groups on the right that both advocate and occasionally carry out such violence. Members of one such group murdered a 9-year old and her father in their Arizona home over a year before the recent Tuscon shootings.
1) Note that this monopoly of violence by the right has not always been the case throughout history (see Russia 1917, etc). However, it is currently a very real dichotomy in the US.
2) Even if one could successfully argue that the people who actually carry out such violence (Timothy McVeigh, Shawna Forde, etc) are mentally unstable and should not be taken as representative of the far right as a whole, in the case where such a mentally unstable person cite as their motivation the violent rhetoric of a political subgroup, all members of that political subgroup bear some responsibility for inciting said unstable individual.