Robert Hunter was born in Scranton, Pennsylvania. Due to genetic alteration carried out on him by the KreeLunatic Legion, Robert gained the ability to explode and reform himself at will.
Some time later, he exposed Captain Marvel to a nerve gas, which ultimately gave him cancer. Since then he has clashed with Earth's superhumans, and been defeated by the "fractioning (separation) of his exploded mass" (essentially, if he can't reform himself after an explosion, he can't explode again).
When Nitro was hired to kill Tony Stark a fight developed between him and Iron Man. Iron Man, whose sensors and telecommunications gear had been recently upgraded, noticed that every time he exploded, a high-frequency pulse was emitted by his body.
Experimenting, Iron Man duplicated the pulse, causing Nitro to explode. After he exploded several times in a minute, he passed out from exhaustion (presumably, it takes energy to explode, and he could only spare so much before he passed out). Iron Man handed him over to S.H.I.E.L.D., along with a recommendation on constructing a null-harness. He was sent to prison.
Nitro was hired as a hitman to kill Matt Murdock, but was apprehended by police.
This explosion apparently killed Namorita, Night Thrasher, Microbe, and the supervillains he was with during the raid, along with sixty children at the nearby elementary school and the surrounding neighborhood in the town where the fight took place. The aftermath was picked up by the superhero teams, who were already split over the issue of superhero registration.
Knowing that Nitro's explosive power could not be that great (at least not under normal circumstances), Wolverine began hunting Nitro after he escaped in the back of a pickup truck. Nitro was found by Wolverine and a group of S.H.I.E.L.D. agents, but Nitro used his powers to kill the S.H.I.E.L.D. agents, and seemingly kill Wolverine. Nitro then took a phone call to an unknown government official who may be involved in the aforementioned Stamford Incident.
After the phone call, however, Nitro found himself in the grip of a nearly fully regenerated, and enraged, Wolverine. As Wolverine pounded on him, theorizing correctly that Nitro's powers couldn't affect anyone or anything in very close proximity, Nitro revealed that he had been taking Mutant Growth Hormone to increase his powers, hence why he was able to cause the disaster in Stamford. Nitro also hinted that there was someone bigger behind the events of Stamford, but before the name was given, the two were confronted by the three mystery agents. They were revealed to be Atlantean royal guard sent by Namor to apprehend Nitro for the murder of Princess Namorita.
During his battle with Namor, Wolverine was knocked unconscious and the Atlanteans took Nitro to New Pangea for torture and interrogation. Wolverine pursued them in Tony Stark's armor and "negotiated" with Namor to see Nitro, only to find that Nitro had killed his interrogators. Wolverine cut off Nitro's arm at the elbow. As Nitro begged for mercy, Wolverine recanted on his promised protection. He allowed Namor to do what he wanted with Nitro, who stayed as a "guest" of Atlantis. His secret benefactor was revealed to be Walter Declun, the CEO of Damage Control.
Nitro escaped Atlantis to the sewers with a few Atlantean terrorists. He was later captured in Latveria and held in Castle Doom's dungeons by Victor Von Doom. Penance later attacked Latveria and forced Dr. Doom to release Nitro, who Penance tortured and almost killed, even forcing Nitro to wear the painful costume. He was found by the Thunderbolts, many of whom were shocked at the level of the injuries. It can be assumed that Nitro was arrested by The Thunderbolts.
Self-Explosion: Nitro possesses the superhuman ability to literally turn himself into a living bomb. Nitro can blow up his entire body at will and exists in a sentient, gaseous form until he reconstitutes himself. Typically, Nitro's maximum blast force is equal to about 250lbs of TNT. However, while taking Mutant Growth Hormone, his power was enhanced far beyond his original limits, sufficient to level the city of Stamford, Connecticut.
After he explodes, he can reconstitute himself at will and can remain in this gaseous form for an indefinite period of time. Nitro also generates a type of kinetic field around his body that also allows himself to reconstitute the clothes he wears at the time he explodes.
During an encounter with Wolverine, it was discovered that if an individual can get close enough to Nitro, less than a few feet, it nullifies Nitro's ability to explode and leaves him as vulnerable as any ordinary human to physical assault. He is also missing half of his arm, which was hacked off by Wolverine.