Having a close look at Kerry King's strumming hand on this picture from Wikipedia :
Why has he a rubber band "pinching" his thumb, index and middle finger ?
What happens if you strum a guitar a lot is that as you get older you may develop inflammation or even arthritis in the joint at the base of your thumb especially if the joint is loose.
It appears that King is using a rubber band to support the base of his thumb to reduce pain.
The surgical procedure to correct this is to take a tendon from another part of the body and graft it around the base of the thumb to increase the support somewhat like the rubber band does here. The problem with the surgery is that even without complications it can take months for full recovery. With complications you might lose the ability to play again.
As a guitar player who's regularly found on stage, I frequently experience cramps in my fingers just for holding the pick, especially when strumming at high speeds. This is because when you're strumming at high speed, you need a tighter grip on your pick, or you'll risk dropping it. Now, I'm no fan of Slayer myself, but I could imagine that the strumming patterns are pretty darn fast. Maybe the reason for the rubber band is to minimize the amount of force needed to grab on to the pick, to reduce the chance of cramps?
I'm not sure about this, but it seems logical to me.
It might also have to do with a case of tendinitis, perhaps. Who knows.