[This is something I wrote in back in August of 2010, but for some reason never published. The recent events in Tucson make me regret not publishing it sooner.]
Republican leaders have gone through the motions of renouncing the threats of violence that have been reported against Democratic leaders during and following the enactment of health care reform, and those Republication leaders seem to consider themselves entirely free of any responsibility for those threats. But those threats are entirely consistent with the rhetoric of the right, including the rhetoric of Republican leaders, and the rhetoric helped to justify violence in the minds of the far right.
Both international law and domestic law recognize times when violence can be justified. The most-well known example in domestic law is the law of self-defense, which allows you to touch, injure, or even kill another person in order to defend yourself against an attack. More broadly, there is a "justification" defense, which allows the use of violence to defend others and not just yourself.
And international law recognizes that a nation may legitimately use force to defend itself against invasions or other foreign threats.
But in each case, the level of violence that can be exerted must be appropriate to the level of violence of the aggressor. You can't respond to verbal threats with physical force, and you can't respond to physical force with deadly violence. In other words, you can't shoot someone who slaps you or shoves you. Similarly, the use of force by nations must be "proportional" to the threat. A country can't bomb a city in response to a border obstruction.
If the health care reform enacted by the Democrats were nothing more than bad policy, bad philosophy, or bad judgment, threats of violence could obviously not be justified, even by the most fervent opponents. But that's not the way the issues were framed by Republicans and conservatives. The health care bill was described as "oppressive" and would deprive Americans of "freedoms." Earlier, there was talk of "death panels." And the procedures used to enact health care reform were attacked as undemocratic and illegitimate. The Republicans described health care reform not merely as bad policy, but as the product of an oppressive, illegitimate government that threatens our life, liberty, and property. They described health care reforms in terms that, if taken literally, would justify armed revolution.
The extremist rhetoric of Republican leaders might not have incited threats of violence by the right, but the rhetoric justified threats of violence. And their defense is that they should not have been taken seriously?