GrimCo
Hero
It's not runing away. It's tactical retreat 
I design world that has inner logic. PC can and do encounter things way above their pay grade and they are told at session 0 that there will be times when fighting isn't best option and that running away is best option. They get enough warnings that going in guns blazing isn't best option. If they choose to do it, well, sometimes it's round 1 TPK, sometimes it's flee for your life, every man for himself. Same for the enemies. Sometimes they will fight till the bitter end, sometimes they will flee after first round.
I handle tactical retreats on case by case basis and use 5e's "rulings over rules" guideline. If they want to flee, i ask them what they want to do and how are they doing it. Sometimes, they might roll. Sometimes, it's pure narration. Usually, mix of both.

I design world that has inner logic. PC can and do encounter things way above their pay grade and they are told at session 0 that there will be times when fighting isn't best option and that running away is best option. They get enough warnings that going in guns blazing isn't best option. If they choose to do it, well, sometimes it's round 1 TPK, sometimes it's flee for your life, every man for himself. Same for the enemies. Sometimes they will fight till the bitter end, sometimes they will flee after first round.
I handle tactical retreats on case by case basis and use 5e's "rulings over rules" guideline. If they want to flee, i ask them what they want to do and how are they doing it. Sometimes, they might roll. Sometimes, it's pure narration. Usually, mix of both.