Every time the Hulk was forced to take a life
Superheroes don't like to kill. Sure, there are darker and more violent types who'd disagree, like Wolverine or the Punisher, but they tend to be the exceptions that prove the rule.
Even in the case of a relatively wild and unpredictable superhero like the Incredible Hulk, killing has never been an easy option. Over the years the intelligence level and personality traits of Bruce Banner's alter ego have changed many times, but even at his dumbest and least cooperative, the Hulk wouldn't consciously kill unless he felt he had no other choice.
But that doesn't mean it's never happened — and a time or two, he seemed to actually enjoy it. Here's every time the Hulk was forced to take a life.