From time to time one of them becomes a copper. He'll get a reputation amongst his colleagues for provoking trouble, never backing down, never walking away and always looking for the slightest excuse to use violence. A constant trickle of complaints and ill-will may result in him moving from work where he has regular contact with the public to the TSG, where he can testosterone-bond with others away from ordinary people. Perhaps even here he'll have a record of poor discipline, a weak ability to restrain himself from the premature application of baton, shield, knees and boots.
Leave him in place, and sooner or later someone's going to get killed.
And just as a pitbull's DNA isn't the dog's fault, the consequences of leaving such an impaired officer in place aren't wholly his own fault. PC Harwood had an appalling disciplinary record, and the risks should have been recognised and acted upon by his superiors long before Mr Tomlinson died.