I thought I read somewhere Karn was a pacifist but he was cracking eternal heads and crushing them. Is it ok because they’re undead or did he stop being a pacifist at one point? Haven’t read a lot of old lore.

Dunno about Karn's vow, but Eternal are mindless upgraded zombies afaik, thus destroying them is comparable to sabotaging weapons.

Karn was only a pacifist during the Weatherlight Saga (before he even became a planeswalker). He gave up on that more than 300 years ago during the Phyrexian Invasion when Tsabo Tavoc and a bunch of Phyrexians were about to drag Gerrard Capashen through a portal to Phyrexia in the Caves of Koilos. Karn decided he was having none of that and started cracking some skulls.

Ok thank you. The way he was going about destroying them was very non-pacifist to being down right brutal lol.

He definitely isn't a pacifist in QFK, but then again we don't talk about that book.