Some arguments

 1. Political assassination has never been good. Obviously, we can lament the killing of Lincoln, JFK, RFJ, Martin Luther King, and Malcolm X. Killing or attempting killing of right wing figures does no good. If you want to look at the killing of the José Antonio Primo de Rivera, it did nothing to stop Spanish fascism, which did not really depend on his charisma. Having him as a martyr was even more convenient for Franco than being alive as a rival. Of course, this was not a political assassination, but an execution by the Republic: all the more convenient for Franco. Arguably, the killing of Calvo Sotelo helped to cause the Spanish Civil War. Arguably, because the war would have occurred anyway. That was just the immediate spark.

2.  You don't want to pick and choose who gets to die and who gets to live. Then you get into arguments about the merits of the people who are assassinated, and not the general principle of the thing. Failed attacks on Reagan or George Wallace promote political violence in general. It has nothing to do with sympathy for Reagan or Wallace, or dislike for their politics. 

3. Generally, the classically liberal principles should be upheld. If we are against the death penalty, against gun violence, against vigilantism, in favor of freedom of speech, then let's stick to that rather than thinking that the sniper's veto is ok,---as long as we can dehumanize the victim enough.