He didn’t murder a child. He killed an active shooter. That’s not murder.
There are certainly downsides, and the risk that he accidentally hits a child in the crossfire is one, or that he identifies the wrong kid and kills an innocent by mistake, are both serious problems with the “good guy with a gun” narrative. But the fact that he killed this particular kid at this particular time, preventing him from killing even more kids which he was actively in the process of attempting, is undoubtedly a good thing.
He saved multiple lives. If your kid was the next one in line when this “vigilante” shot the active shooter, you would be grateful. Anyone would be.