So you might consider slaying the King or not, unless you play Diablo 2 Resurrected, of course.

The Secret Cow Level is a good place to farm Ethereal/Socketed items and low-mid tier Runes and Gems. If you’ve already killed the king or gotten credit for killing him when someone else did, you can’t open the red portal anymore, but you can still enter the red portal when someone else opens it up.

Worse yet, everyone else in the Cow Level, whether they are partied or not, will also receive credit for completing the quest (if they didn’t have it already) and will no longer be able to open the Red Portal. If the character who gets the last hit on the Cow King can open the red portal, then they will receive credit for completing the quest and no longer be able to open the red portal. If they have previously gotten credit for completing the cow quest (which means they can not open the red portal) then nothing will change. Being partied or not has no effect on the credit for the quest, it only matters who actually gets the last hit on King. Credit is given if you kill him yourself, or he dies at the hand of another player who has never killed him before, while you are in the Cow Level.

Diablo 2 cow level guide