15yr-old are not exactly grown-ups. I do not know the circumstances, but even if the kid had a knife, they should probably have tried not killing him more.(maybe they thought he had that.)

They could probably have stepped backwards a bit. Hopefully getting a stand-off or something, or at least some more time to judge the situation. I would expect the police to risk themselves to some extent, because if they would not, they would have to treat everything as a threat, not exactly good for the atmosphere.
Btw this is just yet again a very short and undetailed article we're judging here. Perhaps that is fine, but IMO we are talking about hypothetical cases that might be, and not the case at hand.