Wow, is this ever moving fast. But, for those of you who cannot fathom how quickly a K-knife wielding, boxing-trained person could kill 4 people in 9-10 minutes, well, it happens in combat (and not just the military of one nation, but many nations).
IMO. MOO. Training for this kind of thing is something that some humans do, most of them are not going to use their skill to be a mass murderer.
It's very much like using a gun to kill, if a person trains. So while I am shocked (very) that this method was used here, it is entirely within the realm of known possibility.