Character mistake: When Harry walks up to look in the car which Ricca and his men were shot in, he places his hands on the driver's side and back seat passenger's side window sill, while the crime scene is being investigated. No senior Inspector like Harry would do this. (00:09:40)
Character mistake: The coroner says the pump was shot with a full mag from a magnum which is six rounds but the pimp was only shot with five.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).