No one "Told me" However, there was a huge outcry against a non-Terran player trying to have Terran hurt by the Zuma and it spawned a huge 20+ page thread on how the Zuma should never exist because diplomacy with the Zuma somehow is not worth anything while diplomacy with players is. Thus, I stopped because I didn't want anyone getting mad at me on an OOC level for doing something IC that the game allows. Sure, I know how this sounds but it is 100% true that this happens.
This is not true at all. We didn't try to negotiate with their General. Letters were exchanged, but no actual attempt to negotiate was made, because as you said it was their General and not their ruler. And you did make a good offer, and all of Luria will hate Malus ever more for it. (when they realize)
If you did it, then I would imagine you would get a fair bit of OOC hatred from some people. But then killing a realm does that, running a successful war can do that etc. If you want to ensure no one gets mad with you OOC best bet is to be a silent knight.
Also all of Luria won't hate Malus, Solaria will still loves him. If the rest of Luria doesn't at least despise him he really isn't doing much of a job at being a aggressive self centred tyrant.
