My question is this: in a multiplayer game, is it right that all players team up to curb the strongest and best player? Is it ethical to create temporary alliances? What is the boundary that separates a tactical alliance from manipulation?
All of this is definitely part of the game and to be expected in a free for all multiplayer format. If you don't want to get ganged up on, then you need to pace yourself and not play every threat you have, just because you can, if it won't cause you to win on the spot. Managing your own threat-level is an important art to learn.
edit: regarding boundaries, as long as all agreements are kept in spirit (as well as to the letter), I'm fine with temporary alliances, even if in hindsight they benefited one player more than another
Another problem is that of the "paternalistic spirit". Whenever I cast a spell, it's always the wrong one. The other players expect me to line up alongside one of them. They do not accept that it can have my own independent strategy. How can you play magic in this unbearable climate?
Call them out on this. I've been told I did something like this in the past (without me realizing it) and since then I've tried to do better. The people you play with might need a similar wake-up call.