It isn't lust seal, meat in general is on the menus, just seal is most common where Orca are found. There isn't much in the way of vegetation to eat in the ocean, and they don't have huge mouths like other wales so can't eat krill with much efficiency.

They are hungry. Although I've also read that orca "enjoy" hunting seals, kind of the same way some humans enjoy killing deer and other game.

It's big, it's there, and it's slower and less maneuverable than fish.