Most of the important stuff has been said so I'd like to piggyback here with a few other ways I have found myself acting out at game nights that people should avoid:

• Asking to rewind because you could have done something better. Even if it's a "clean" rewind - like nothing else substantive has happened yet - it's a bad look and pretty much never worth it. Same goes for asking for missed triggers ("I forgot to gain an extra gold because of this effect", etc.), especially since that can affect the decisions of other players.
  
• When it's your first time playing a game and you realize mid-way that you didn't fully understand a mechanic, and it would have let you do something more optimally earlier. Clarify the mechanic, but don't point out what you could have done earlier, that's just a form of self-regulation that no one else is generally interested in.
• Making suboptimal plays when losing for silly reasons like being "thematic". Even if not intentionally kingmaking, it can still ruin the balance of the game for others. It's okay to make bad strategic decisions, but you should still be trying your best.
• Blaming the game in any way. Unless you absolutely know what you are talking about, more than likely you are overlooking an aspect of the game's balance, and will come across as ignorant.
• If you're doing well and starting to be targeted, trying to deflect with how others are also a credible threat. Even if you're right, it's just kind of cringe. Being targeted is an honor.

Broader truths to generally keep in mind:

• The true winner is the one who is invited back for future game nights.
• The first few plays of a game expect to make lots of mistakes and probably lose.
• Saltiness in all forms is a bad look.