I don't know much about PUBG, but I did play a lot of other online shooters. Based on those experiences it seems there's a fairly large number of players who thinks that anyone substantially better than them are cheating.

They played with poor sound setup so didn't realize I knew exactly where they were thanks to a good headset and could blast them in the face as they rounded that corner.

Or they didn't know about drag/flick shooting[1] so they thought the snap that landed the headshot was a cheat rather than a deliberate, physical mouse movement.

Or just they just significantly overestimated their own abilities, thinking anyone being that much better must be cheating.

[1]: https://themeta.com/aiming-essentials-3-flick-shot-technique...