I see it as a sliding scale - SC is further up the moral scale than that Iln that went on a indiscriminate rampage against defenceless beings, but they still take very violent and immoral liberties against many lesser societies within the Culture's sphere of influence for the greater good, often carrying out assassinations and causing wars. They threatened to cross the line a handful of times (like that E-Dust drone conducting that grisly and lighting fast hit on those Chelgrian conspirators in the epilogue for Look to Windward).
The Iln may have had noble intentions, but in their case they crossed well over the line and their power trip of being a on self-righteous quest of liberating the galaxy from the Veil with their Culture-level weaponry may have fatally corrupted them, with the Culture being socially more advanced than the mainly extinct Iln, who likely did not socially keep up with their HLI technology. However the depraved, murderous actions of the Iln device could be the result of it being a incomplete being that was driven insane after millions of years of isolation, since only three out twelve (?) smaller parts of the Iln device were reunited with it, so it did not have all it's faculties (like moral oversight). What if you took out giant chunks out of a Culture Mind, that included the more humane and restrained parts of it's personality, then locked it away for twenty million years?
Matter prologue online
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