Mega Evil Empire - Triggered once or every turn?

[Q]

Welcome to this forum Steppenwolf und ein herzliches "Servus" nach Wien!
I believe the anger increase is by turn, because I observed several times that not all AI would declare war at me at the same time. Races which stated at a "brotherly" mood took more than one turn to get "murderous". And as you said if the anger increase was only once, you should be able to get some races friendly again. I did never see that.
But you can test this yourself: change the value in the AI anger file for "Mega Evile emire" from 50 to ten. It should take much longer till the AI get murderous after you reach te MEE level, but eventually they should get angry unless you choose a very low value of about 2.