You're going to inject a druggie with tranquilizers?
Aside from the substance inter-action issue, tranqs are not magic. They do not work instantly, and I don't know about you, but if I'm shot with a tranq, I'm likely to be considerably more aggressive in the immediate aftermath - and I'd argue that induced intoxication can certainly be a mitigating factor in whatever I would do after that point.
Obviously the man being struck didn't think the hitting was unnecessary...he kept fighting.
That's the point. A drugged up person is going to keep coming at you. Even a shot to the foot would be better than beating somebody to death, don't you think?