Technically Illithids are only evil in the eyes of other races. This does not make them actually evil, as you said they do not have a similar moral framework to other races.
Well... evil is an objective measure in D&D (not talking about IRL philosophy here). An evil person/race doesn't need to consider themselves evil or go against their code of morals to be evil. The 5e player handbook says:
"Lawful evil (LE) creatures methodically take what they want, within the limits of a code of tradition, loyalty, or order." Which fits Illithids the best imo.
