One of the big questions that has plagued the Marvel universe since it's inception is how the Hulk can be considered both a hero and a monster. A theory that answers this question is actually explained in The Immortal Hulk, specifically issue #20. It was written by Al Ewing and illustrated by Joe Bennett.
A big part of the series is how Bruce Banner develops a dangerous persona that he called the Devil Hulk. During this time, Rick Jones's body was taken and used as a basis for a whole new Abomination creature. Also during this time, Banner's ex-wife, Betty Ross, was shot in the head and transformed into the monster known as Harpy. Together, the Devil Hulk and Harpy took on this new abomination and managed to kill him by ripping his heart out and dropping him from the sky.
Shortly afterward, the military group responsible for creating this Abomination shows up in two heavily armored flying tanks to finish the job. The duo take on and destroy these tanks, too. The Hulk even destroys one by throwing it into the other, creating a huge explosion that no doubt killed the driver. At this point, the reporter that had been following the Hulk mentions the theory by Amadeus Cho. It says that although the Hulk causes trillions in property damage, no innocent civilians die during his rampages because Bruce Banner was working subconsciously to stop the Hulk from killing anyone.
The Devil Hulk responds that he has actually heard this theory. He thinks it's weird but he also says that it makes sense, just for the other Hulk personas. Unfortunately for others, it doesn't work for him as the Devil Hulk is more intelligent and ruthless than some of his more mindless brute forms that can barely speak. But technically, the theory still stands as the Devil Hulk didn't really kill any civilians during the fight. He only killed the drivers of the tanks in self-defense.
Applying this theory to the entire Hulk's career within the Marvel universe, it shows that he can be both the hero and a monster at the same time. His character doesn't have to be just one or the other. Similar to the fact that the Hulk is actually two people at once, Banner and the Hulk, he is both a monster and the hero as Banner works within Hulk's mind to steer his heroics while still being unable sometimes to contain his rage and destructive tendencies.