You say grudge. I say reasonable aggravation. Archive attacks a Relorian planet with no provocation or advance warning. They simply show up with their fleet, weapons already trained on the planet. Archive doesn't even say a word to them before firing their first shots, then proceeds to not give anyone on the planet a choice, or even a valid reason. Destroying a planet just to find one person? Pathetically absurd and absolutely one of the most inefficient ways of going about it. Hell, even holding the place hostage would have been a better choice. But no. The Archive decided to completely blow off all forms of reason or sense, and destroyed the entire planet. And now, they have the balls to complain about Reloria having a grudge against them. I don't care who you are, that'd piss you off too. Yeah, they had enough sense to allow some of the Relorians to leave. They did not, however, give that chance to any of the natives. They did not give anyone enough time to gather their things. They did not grant anything living on the planet a chance. That, due to their reasoning, is unforgivable to the Relorians.
The only reason the Archive hasn't been branded Kuo like the Starclan was because they allowed some of the Relorians the chance to leave. That's it.