The shark is often killed already prior to the liver being eaten. If it isn't, then it would certainly die very soon after. The shark liver is one of the most calorie dense things in the animal kingdom and accounts for a quarter of the shark's weight. Orcas eat it because it's fairly easy for them to kill sharks and the calorie intake. Orcas are much bigger, faster, stronger, and smarter than sharks. They do it for survival.
Not to mention that the rest of the shark is perfectly recycled by the ecosystem.
What options does an orca have? How is it cruel that they eat a shark? Have you ever even seen the cow, chicken, pig, etc. farms that are in use today? It's not like these animals are living in fairy-tale meadows all their life before they're killed.
And no, if everyone ate cows that way, they would not be perfectly recycled by the ecosystem. These out of context arguments are never useful because they sideline the facts, the scale and population of humans being the relevant ones here.
Not to mention that the rest of the shark is perfectly recycled by the ecosystem.