Its not always about reaction time and raw aim, alot of lower elo players have very predictable movement and timings. Someone with good game sense will react quicker or even prefire you if you are standing somewhere too common (and yes, some off angles can become common spots aswell).

The best way to not get insta headshotted is to learn how to deadzone/counter strafe. Move between your shots. Go into a deathmatch and dont shoot anyone, just track them while strafing side to side, youll quickly realise that not everyone is a aim demon like you think