If person A commits an act which person B finds offensive, would person A be morally responsible should person B exact revenge on person C? No. Would person A be personally responsible? Nuh-huh.
But, what if person B reasonably believed that person C was equally culpable for the original action as person A? What if, in the absence of person A in their vicinity, person B decided to – let us say for the sake of argument – shoot, hack or burn person C to death. Where would person A stand viz. moral and/or personal responsibility?
First consideration would be if the proposer of this gedankenexperiment believes shooting, hacking or burning to death to be a suitable judiciary punishment. If not, responsibility should be placed with person B. Yet, this should not preclude it also being placed with person A if causation can be shown between person A’s actions and person B’s response.
Say, person B was a violent and paranoid schizophrenic who believes that fluoridation is a plot by Princess Diana to poison the water supply, and person A was seen with a tube of Colgate. An explosive response could be expected, but would person A be morally/personally responsible?
If they had purposefully gone into person B’s house to do so, some charge of crazed indifference should apply. Not so if they were using Colgate in the privacy of their own house.
Let us now say that the proposer of this gedankenexperiment would not be asking these questions if the roles were reversed for persons A and B. Would that make them as big a tool as Matt Seaton?
Update: Okay, let us try another gedankenexperiment. There is no person C, and person A has committed some act of extreme violence against person B or their kith and kin.
Would person B be justified in hold a public protest calling for judicial sanctions against person A? Yes.
If the proposer of this gedankenexperiment has similar views to the proposer of the previous one, would it be reasonable to assume the current one believes that person A would be morally responsible for any violent confrontation? Yes.
In this case, is person B calling for violent retribution to be enacted on person A or anyone they believe is equally culpable; instead they are, let us say for the sake of argument, merely blocking a thoroughfare. So that question is moot.
Would it be reasonable to anticipate a hostile response from person A or their kith and kin? Yes.
If this occurred, would be be reasonable to frame description of events in terms of a “clash between [persons A and B]”? No.
Is Matt Seaton still a tool? Yes.