Everyone is armed all the time and that’s normal, but to draw a weapon is an overt hostile act. A standoff therefore is a game of chicken because both want to kill each other and you want to draw first to have the highest chance of surviving, but even a bandit will hesitate to add a felony murder charge to their rap sheet. The whole town serves as witness when there is a pair of eyes behind every shuttered window. The hero always draws second, both demonstrating his superior skill and speed by defeating the opponent even at a disadvantage, and getting away with murder scot-free.
This lacks a lot of context as both strategies are useful in different circumstances. For a player (A) holding an angle from a far distance, this is true because the person coming around the corner (B) from a short distance to the corner is visible to to A before A is visible to B. This gives the reactor an advantage. This is usually afaik called angle advantage
For a person who is close to a corner peeking a person close to the corner, the opposite can be true due to the delay it takes for the movement to be relayed to the other player’s computer. This means that the player peeking may in more extreme cases have a 100-200ms advantage over the reacting player. This is dubbed peeker’s advantage.
The effects can vary with latency and distance,