(b) The force used being proportionate to the danger
In that case John wasn't attacking anyone at the monent, so no defence. The 4 shots are also way disrpoportionate to the non-danger. And they came within 5 seconds of ordering to drop his knife, which again is not enough time to react (also assuming he could hear it).
In this case the shooting is more of an illegitimate action of offense.
Note that police doesn't just do actions for defence, but also e.g. to enforce some law. But the shooting, 4 times, lethally, of someone away from you, carrying a knife, within 5 seconds of you having told them to drop it, is not justified even under this context. At least it wouldn't be acceptable in any civilized country.
