My favourite “good guy” ending was during my 2nd playthrough as a half-orc berserker with Lae’zel, Shadowheart and Wyll in my party. I severed Wyll’s contract and saved his father so he became Blade of Avernus, turned Shadowheart from Shar and made Lae’zel abandon Vlaakith for Orpheus. I of course proceeded to murderize our favourite charming cambion and acquire his hammer, freed Orpheus after telling Emperor to fuck off, like I have been for the entire game, and turned myself into a mind flayer. I killed the Netherbrain and at the end, after Orpheus gave me his heartfelt thanks and flew off with Lae’zel to free their people, me, Wyll and Karlach went to Avernus to kick ass together.
The respect both Orpheus and Voss shows you for sacrificing yourself to give their people a chance for freedom really makes the choice worth it, and going to Avernus with Karlach gives you plenty of guilt-free brains to eat and even if you do ultimately loose yourself to your new form, at least you are trapped in the Hells and away from Faerun.
So does Fireball and yet that one does no friendly fire damage. Either Ice Storm is bugged or it’s as the other commenter pointed out, Guardian of Faith detected a hostile action and triggered an attack on all nearby hostiles (your party), which dealt enough damage to it to kill it.