When did Val Kilmer's career downfall start?
I guess I'm somewhat concerned about him because he has been looking really sick lately do his issues with throat cancer. But that's another story. It seemed like Batman Forever marked the pinnacle of Val Kilmer's career (or the point in which he was officially A-list if he wasn't already). It seemed like Red Planet in 2000, was Kilmer's last real shot or opportunity at headlining a big budgeted major motion picture.
Personally if one were to point out where Kilmer's downfall began, it would be The Island of Dr. Moreau. It was an awful movie in its own right, but it was Kilmer's off screen antics (such as what happened between him and Joel Schumacher on the Batman Forever set, hence why Kilmer didn't play Batman more than once) at this point that really started costing him roles. During filming he burned one or more screen members with cigarettes, and was often boorish and difficult. You can get away with acting like an ass when your movies do well at the box office, but once they start flopping, no studio is going to put up with that garbage.
https://lebeauleblog.com/2011/05/05/what-the-hell-happened-to-val-kilmer/