Surprise! If this person is already in a relationship with someone else, they can’t be in a relationship with you, too. If you’d known this ahead of time, you wouldn’t have shown up in the first place.
There’s nothing wrong with having a drink or two to relax your nerves, but if it seems like your date is more interested in doing Jager-bombs than having an actual conversation with you or their alcohol consumption is making you uncomfortable, it’s OK to bounce.
They read a book about “negging” and think that subtly insulting you is going to make you more attracted to them. Wrong! If someone criticizes your appearance, interests, weight (“If you lost a few pounds, you’d be a really hot plus-sized model”) or says anything else that leaves a really bad taste in your mouth, #bye.
You can’t quite put your finger on it, but something feels off. If you get a creepy vibe or generally feel uncomfortable with this person, you don’t need to justify it. Trust your instincts. Make an excuse and get out of there.
Never compromise your standards, well-being, or moral compass. If you feel uneasy or unsafe at any point, never hesitate to leave. Don’t feel guilty or bad about it and know that you’re better off.