According to the PHB when you roll a 1:
“If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target’s AC.” (p194 of the Player’s Handbook).
Which is frankly boring. A critical hit or fumble opens the door for stakes, character development and story to be told. When there is the opportunity for something unexpected or challenging to happen to both PCs and NPCs it makes the game more dynamic.
For transparency and fairness, we will be using the same charts for PCs and NPCs, and all with have the opportunity to access them.
https://sterlingvermin.files.wordpress.com/2016/09/critical-hits-revisited.pdf