WineGod, for what's it's worth, I think I know who you're talking about and I have a more positive perspective on them. When I first started playing, this player took quite a bit of time to orient me to the rules, the site, the general culture, etc. This was in a game that I was the SC leader of for much of the game, and I ended up winning that game, so I know this wasn't some elaborate sweet-talking ruse. Of course I don't know what exactly went down between you two, but just wanted to add that I've had positive interactions with them.
Speaking of toxicity, this is a bit unrelated, but I think all of the "unwritten rules" of diplomacy are really fascinating. Almost every player has some sort of threshold where they define a boundary between "crafty but honorable gameplay" versus "dishonorable/unsportsmanlike gameplay". I think diplomacy is kind of similar to pickup basketball where there aren't any referees, so in order to keep any norms at all, players must self-police (there are mods that can monitor for outright cheating, but no "referees" to define unsportsmanlike gameplay). And all of this is in the context of wildly different definitions of what is acceptable vs unacceptable gameplay--e.g., some players (including me) think it's important to continue previous alliance when taking over a CDed nation; others re-evaluate alliances pretty ruthlessly. This is a standard of unsportsmanlike gameplay that has to be policed by players, not mods.