It will probably give some libtard the vapors.
While it is too bad that the perp survived, I can think of a number of reasons for favoring smaller caliber ammunition including:
Lower cost = more practice time.
More accuracy in a high-stress situation = less collateral damage.
Less attention from the ammunition nazis.
Problem is that those reasons will get you killed.
In the case under discussion, the bad guy was able to shoot and kill the victim.
We don’t know why he did not, but the victim was very lucky that he did not.
Possibly the face wound restricted his vision or caused him to panic. Or maybe an arm or shoulder wound disabled his shooting hand.
As for the shooter, type of ammo is as important as caliber. Many uninformed are carrying cheap ball ammo which is only suitable for range practice.
There is ammo available that will make a 9 mm effective.
I like ammo that makes big holes and starts with .4.
It will probably give some libtard the vapors.