Watch your Mercenary and Summons as they can 'accidentally' kill him, too. If you don't want to complete the Cow King quest or are unsure, the safest place to be is in town (use Town Portal scroll!). If anyone kills Cow King for FIRST time (except, a person who has NOT killed Baal/Diablo, see below), then everyone INSIDE the Cow Level will get credit for the (hidden) Cow King quest and will NOT make the red portal anymore.
