To be clear, you believe that, right wedrifid? I came this close to downvoting before I deduced the context.

I believe that there are times where the described behaviour is morally acceptable. I don't think it is helpful to label that behaviour 'murder' but if someone were to define that as murder it would mean that murder (of that particular kind) was ok.

To be clear, there are stringent standards on the behaviour which preceded the mistake. This is something that should happen very infrequently. Both epistemic rationality standards and instrumental rationality standards apply. For example, sincerely believing that the person had committed a crime because you happen to be bigoted and irrational leaves you morally culpable and failing to take actions that provide more evidence where the VoI is high and cost is low also leaves you morally culpable. The 'excuse' for hunting and down a killing an innocent that you mistakenly believed was sufficiently evil is not "I was mistaken" but rather "any acceptably rational and competent individual in this circumstance would have believed that the target was sufficiently evil".