Look, I do understand that you waited a long time to sing tonight. You weren't the only one who waited a long time to sing tonight. Unfortunately, we ran out of time and were not able to get to you. You weren't the only one we weren't able to get to. You were, however, the only one who was a cunt. You and your friend. So you want to whine to management about how "bad business"-like it was for us not to allow you to sing because we ran out of time? Be my guest. If you've been participating at karaoke for longer than I've been working, I'm sure you would know how things work on busy three-day weekends. But I somehow doubt that you're a regular (as I am a regular, years before I started working there, and don't recall ever seeing you there).
So, please, cry to the general manager about how unfair I was tonight. But realize that you are the only one who reacted this way. Everyone else seemed to understand that it was an extremely busy night and didn't get up in my grill over it.