The form of “revenge” here shouldn’t be viewed as a form of maliciousness that extends beyond just the competition of the game. Making a threat of retaliation, and even following through on it, is a potential tool to influence other’s play, not to harm them on a personal level. However, there is definitely a psychological aspect to it as well, in whether such a threat seems credible to actually work.
It’s important to also draw a psychological sandbox around these games. Even though a game of standard Go might be described as “fighting”, “violent”, “battle of life and death”, etc., we quite easily view that abstract competition, which we ultimately view as a joint exercise in recreation rather than actual conflict, as something separated from how we interact with that person outside of the game. With Diplomacy, however, it can be bit tricky to naturally view things that way, since the game revolves around interaction via language, yet, we still must understand that the concept of retaliation and revenge should not be an interpersonal attack, but rather part of only the artificial conflict of recreational competition.
Related to such concerns is how much psychological factors may influence the interaction between the players across multiple games. Ideally, each game should be a clean slate, and generally the rules for a series of games include prohibition of any explicit alliances, deal-making, grudges, etc. that carry across games (i.e., no one should offer a quid pro quo or make a threat that spans two separate games). However, how the psychological biases and reputations between players might carry over across multiple games is a much grayer area, and anyways very hard to cleanly define or draw boundaries around.
One could certainly argue that some of the discussion in this thread, such as expression of one’s intentions, preferences, and general strategic thoughts, falls within the realm of building reputation and revealing intentions/preferences such as to potentially influence future games.