As far as I can tell, it's a necessary consequence of any mention of firearms on HN, in any context. There seems little point in blaming any one participant, in any one instance, for the perennial argument over what is perhaps the most immediately, or at least the most reliably, polarizing issue ever raised on HN.
Others include Isreal / Palestine; sexism; RMS; etc. it is trivially easy to troll HN. HN does not do well at ignoring trolls or not responding to overly-enthusiastic posters. (There are some exceptions to this where a person will carefully talk through differences in opinion to try to understand or communicate a point of view but these are exceptions).