Does reading strategy guides before a game night count as cheating?
This is all about your social contract1 - some groups play to win, in which case the behaviour you've described seems perfectly reasonable. Other groups play more for fun, and fun can often be reduced if one player is significantly better at the game than everyone else.
I'd wouldn't go so far as to call it &quot;cheating&quot; as you didn't break any of the written rules of the game, but it's obvious here that your friends had less fun because of your actions. At that point, it's behaviour that should be avoided.
How to avoid it? You already know the answer to that - talk about it beforehand, but with your group not with the Internet. Whatever you agree with your group that lets people have the most fun is the right answer (and sometimes that may involve people finding that they would have more fun if they played with a different group).
1. Link to Role-Playing Games SE, so obviously refers to some RPG-specific concepts, but the concept of a social contract applies across any group setting