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Kustard, good question! I have watched the final episode, before I read your post but the murderer, Scott, didn't go on the rampage because of the miners' strike or the fallout from that. The community assumed that was the reason and that padded out the storyline with fresh tensions between the strikers and the scabs.
It was nothing to do with the spy cop, either. That was another red herring that gave an interesting plot to the series.
It was simply because Scott is a twisted human being, jealous of close families and he didn't like Gary. He felt like an outsider, somebody ignored and was resentful about his upcoming conviction for benefit fraud - he claimed he didn't know he was supposed to declare his savings, and was angry at what he saw as an injustice that would blight his future.
He chose his targets to muddy the waters in the hope it would give him a chance to escape.
I enjoyed it.