Killed, you say?
Although the person in this video SITS up, from a laying-down position very closely aligned with the location of Khan after being shot, you do NOT see him GET up.
Consider the length/angle of the shadows in this post's gfootage of the 'take-down'.
It is not long after the 'take-down' captured in the video on this post, which appears to have been shot from the upper deck on the near-side of the road.
Shadows in both videos suggest footage taken at similar time of day?
When was the last time you saw a reasonably dressed vagrant take an afternoon nap on Westminster bridge, with no foot or road traffic in sight?
Consider the scene SHORTLY AFTER the 'take-down'. Secured by police on-scene, with public pushed back to a cordon at a 'safe' distance (They've used this scene for such 'theatre' before, yes?).
Consider the footage in the vid of 'Mr Sit-up'. Evidently shot from across the road, and through an above ground floor level window.
Consider his attire, compared with Khan's attire in the attached vid.
The footage does NOT show him get up and walk away (where did you see that?).
An instruction to lay back down, so that they could damp-down interest and clear-out bystanders, then let the press grab some press-shots for aftermath effect before you wrap up the day's 'shoot'.
Tell me, that you engage in critical thinking and review all available footage/evidence (as close to primary sources as is practical for your means).
No need for apology fren.
Should point out, I meant 'London Bridge' (not Westminster.
Checking streetview, looks like there's a multi-storey gym on the opposite side of the road, from the take-down.
Most likely location of 'sit-up' footage was shot through a window from there. Being shot through glass on a mobile phone, may explain some loss of colour definition.
It doesn't look like fake footage to me, so I won't readily dismiss it. I would love to get hold of the full video, as this is just a replayed/commentaried segment of a longer video. A forensic analysis of that would likely yield gold, if it be there.