So this is no longer about the events of this war in specific? The Thargoids have just been damned since the events of 500 years ago in your view, and that justifies us unconditionally? Would you accept the opposite if it turned out that humans had actually been the first to shoot 510 years ago, so the Thargoids have rights to do whatever they want to us?
Recent events are evidently not connected to those from previous conflicts. If they were, well we already had a first Thargoid war, they would have returned on a war footing from day 1: not doing what they did, months of strict pacifism replaced with targeted retaliation when that failed.
The 2800s is even more murky. What makes you think there was a "murder spree" as opposed to Thargoids defending themselves, if indeed these disappearances were actually to do with Thargoids? And why, if these events are relevant to today, why do you think they started acting so differently for the events we definitely know were them?