Better to just execute them on the spot - you don’t want them slipping out of the cuffs, then have your partner’s widow asking “why didn’t you just shoot them?”
People are already searched upon arrest, and I don’t think most people are inclined to try to fistfight a pair of cops armed with batons, tasers and guns from the back of a cop car. It’s not necessary in most of the rest of the world - are the residents of the land of the free so inherently prone to violence that they should be locked up just in case?