I like the comparison of guns with ideas, because both are potential weapons.
Ideas clearly cannot be held responsible directly, in the sense that a person espousing them can. The apportioning of blame between people and ideas, for atrocities that result from said people acting on said ideas, is a labor of Hercules.
The argument can be made that without the idea, the person would not have acted as they did.
Yet the argument can also be made that a person inclined to commit certain acts would find ideas to so motivate them.
When you bring in atrocities committed on a grand scale, such as the Nazi Regime's Holocaust, you're throwing additional ingredients into the pot. The ideas espoused by Hitler were spread to so many, not solely [or perhaps even primarily] on their own merits or because all of those people were inclined to embrace them, but because of Hitler's charisma.
Ideas are one branch of a virus; take out that branch, and you have a good chance of killing the virus.
In short, I believe that ideas are partially responsible for the actions of those who embrace them; they are not necessarily protagonists or antagonists, but they play important supporting roles.