Murder ballads are socially useful.
People gossip about people they don't know in order to socially construct moral norms. This is why the culture digs into the personal lives of musicians or actors whom we only know from their unrelated professional work. It is very socially useful to have examples of their relationship drama laid out in public so that we can all articulate how we feel about what Correct Behavior in these different situations would be. This is not so different from Jesus speaking in parables, or pastors telling stories in sermons. (it gets horrifyingly lowest-common-denominator and mob-mentality in its new incarnation on AITA...)
Anyway, the social function of chewing over all this together is mostly to decide how we're going to think about things either generally or in our own lives, with only tangential connection to the persons discussed. The significance of the reaction to this recent murder isn't "[this encourages people to go out and murder healthcare CEOs]." A: I don't expect people to do that. (They shouldn't, but also, they won't.) But B: that's not even the part I think really discomfits commentators. This act is an anecdote that did not need to have actually occurred to fulfill its function in the discourse; it could just have been a thought experiment people agreed to all talk about for a while. What does justice look like? What does injustice look like? Who is supposed to have a monopoly on violence, and for whose benefit have they wielded it? Some people seem really uncomfortable with mass engagement on those questions, inseparable from their own answers not being so satisfying to the masses...
Vigilantism is bad! Just to be really clear on this! But also, if we're constructing systems of power where many people seem to have to squint pretty hard at the moral equilibrium involved here to get to that conclusion, that says something pretty loudly about those systems that is worth the attention.
So IMO? It's good that be done with harmonicas.