McGurn was not one of the Valentine's Day massacre shooters. He was holed up with his "blond alibi" in a hotel room, constantly ordering stuff from room service, before, during and after the shooting to establish an airtight alibi. The shooters were Fred "Killer" Burke, James Ray, Joseph Lolordo, Albert Anselmi and John Scalise.
Capone didn't initiate the massacre--McGurn did. He was seeking revenge on Bugs Moran because two of his gunsels, the Gusenberg brothers, trapped McGurn in a phone booth and shot him up, nearly killing him. Capone fronted him some money to set up the massacre. The vast, nationwide revulsion that the massacre caused brought a lot of heat down on Capone. To get out of the heat, he had himself arrested in Philadelphia and spent a year in prison. McGurn quickly lost favor because of that.
A much more trustworthy explanation than the usual crap that McGurn "fell out of favour due to heavy drinking and that his "hit-skills" wasnt needed anymore in the 30s." Eeh, the Outfit was more vicious than ever at this point. Very interesting and not in so very many books.
