In my long gaming “career” I’ve played under some truly great GMs and some truly terrible ones. Being a great GM is a time consuming labour of love (unless you are one of the special ones) as you labour over creating an adventure to enjoy with your players. Being a terrible GM is easy.

Simply:

Treat the game as a competition between you and the (hapless) players.
Arrive late; declare loudly you haven’t prepared (blame someone else for this) and then run what amounts to a series of boring random encounters.
Favour one player over the others.
Cheat.
Ignore what the players want to do or where they want to go. They WILL experience the adventure as you have designed it (or only do the bits you’ve prepared).
Kill a character if the player is annoying you.
Make petty judgements about time, equipment, the terrain and so on to prove your superiority.
At the end of the session, tell the players what they did wrong and go over at least one of the combats in excruciating detail while pointing out the players’ tactical errors and the like.
Interrupt play to tell stories that have no bearing on the game. For bonus points, make the stories about you and previous adventures you have run (amazingly).

Be oblivious to your players’ feelings. You know best.