Yeah, so, if the Hulk went nuts? Goodbye, world.
Seriously, it's hard to imagine many things more potentially catastrophic than an evil Hulk. Sure, the Hulk we do know has often caused some pretty severe property damage in his rampages, but that's more incidental: he's either trying to defend himself from
military attacks, or looking to take down a bad guy. However, just imagine if the Hulk truly became a villain — an unstoppable monster that only became stronger the angrier he got.
For an example of what would happen if the Hulk went evil, take a look at the comic storyline "Future Imperfect," which introduced a bearded version of the Hulk from an alternate future who called himself the Maestro, and decided to take on the world. Needless to say, the world didn't survive him too well. Let's hope evil Hulk never happens in the regular Marvel Universe, or it's toast.