Supposing the police acted essentially reasonably in this case (as I understand it, the victim was certainly brandishing a knife and ignoring instructions). But still, are you not troubled by how many people meet US Police and end up dead? Even for things initially as small as traffic violations. You can go on about how "he might be a threat" all you like -- the fact is that far, far too many people end up dead when meeting the police, whether they committed a crime or not, whether they had a gun or not, and whether they complied with instructions or not.
It should be troubling indeed, and yet all people seem to do is blame the victim, fail to spot the pattern, and show not a hint of concern that this shouldn't be happening. The Police are not Imperial Commissars.