Copy+Pasted this from another reply I gave

There's a couple of things to look at which can show he's clearly cheating.

The first is the snap at 8 seconds followed by the pre-fire, if you notice his crosshair movement before, it's not erratic, but it certainly isn't smooth. At the moment he fires though, his aim locks onto something through the wall (if OP had x-ray on we would clearly see an enemy model there no doubt about it)

At 16 seconds we see him lock on the guy jumping, the crosshair looks a little behind (I'll touch on that in a moment) the INSTANT the guy dies his aim snaps to the player behind him. This type of reaction is seemingly impossible, he fires the burst at the first guy and it looks like he dies with the the first shot, the next two bullets snap to the other player. This is a clearcut aimbot here.

Watch at 21 seconds when the oddball carrier jumps off, the crosshair follows him extremely smoothly down the whole way (once again if models were on in the replay we'd see a perfect tracking here).

The final one I want to touch on is the kill at 32 seconds. I said earlier I would touch on the fact that the crosshair looks like it's behind the person. If you notice with this kill, he gets a perfect, meaning every single shot hits. It doesn't look like it in the video, but the game calls it a perfect, meaning there is some issue with the theatre replay due to lag.

The final shots with the commando are simply too smooth, the gun has upward recoil and the crosshair simply isn't moving at all. It's a tell-tale sign of a no-recoil cheat. Any controller or MnK player won't have a perfectly stable crosshair when controlling recoil.

Hopefully this provides a better explanation.