GE_EntityKilled & GE_SquadKilled do not contain information about the killer owner or killer entity/squad when the unit dies at the same time.
This is big problem that occurred multiple times during Outback Octagon 2 that made it impossible for mod to detect who killed the King (which was big deal as killing king granted extra population)
Example how it can happen:
Unit A is killing Unit B. Unit A makes final attack and is registered by the game. Before Unit B dies another unit kills Unit A. → Result is now dead Unit A finishes of Unit B. Both event fires without information that could be used to determine who killed Unit B.
More common and easier to replicate is to have Demolition ship to kill other unit or building. Whenever this happens game does not recognize this case. This means also non-modded games suffer from this bug if they are rely on the events to provide the Killer or Killer Entity/Squad.
This event bug is also related to Monk Conversion turning units neutral, when Monk dies at the end of conversion.
Suggestions: update event to always include killer (if it was player owned unit at any point of time, including if it’s at the moment neutral, due to player being eliminated or something) or killer entity/squad.
Otherwise provide guidelines how to avoid this issue in all possible edge cases with 100% correct detection. Detecting it based on damage received is not consistent from my observation and I am not aware of any other way.