Heroic sith raid

If it was from Nihilus' Annihilate ability, everything's working as intended. GLs are immune to instakills from normal toons and other GLs but not by Raid Bosses. It's in fact written in each GL's Unique ability ("They take reduced damage from destroy effects (excludes raid bosses)"). The only exceptions are Rey and SLKR when they're in their Ultimate stances. In other words, against regular Nihilus, they'll survive the Annihilate and instead just take massive damage (which is then further reduced by that same Unique ability I mentioned), whereas against Raid Nihilus, they get eaten alive.