And add to the fact that it was probable that the same criminal had put out a hit on four people, two of whom are teens and one a toddler. I dont know anyone who is a cop, but I feel that they would feel the same way Alan does. One less would be child killer off the streets aint a bad thing after all.
Murder is murder, no matter who the victim is.
Murders in prisons are also investigated. If Sorel had been mugged, Mac would have had a legal obligation to investigate that as well. In general, I doubt that IRL detectives would have such personal knowledge of Sorel. Probably they would go thru the motions, but without much enthusiasm. I think a certain amount of investigation would be required.