If the organizer was advocating use of firearms, or was inciting violence in general, and the handgun just happened to be nutjob's weapon of choice, then yes, the organizer should be liable civilly if not criminally.
If the organizer scheduled a speaker he or she knew or should have known would incite similar stuff, yes again.
If the organizer scheduled a speaker he/she had no reason to believe would incite violence, but the speaker surprises everybody and does so anyway, and the organizer makes no attempt to stop it, yes again.
But I would be shocked if any of that actually happened in the real world.
Absent that sort of six standard deviation, off-the-charts behaviors (IOW 99.9999% of cases), no, the organizer should not be personally civilly or criminally liable for actions by miscreants in the audience.