Now all of you are questioning me, "Why did Omni turn to corruption?" "I mean he was a pure person, what changed?".

Well it comes down to this:

The villains in his world all were redeemed and turned into heroes, but Omni couldn't accept this, as he thought that turning all villains into heroes was a pretty bad idea. You see the villain that is a true threat to society/life/reality didn't exist or rather was already defeated by the previous heroes.

Union in Omni's eyes was the last true villains to exist and he liked that.

You see Omi despite being pure was secretly obsessed with villainy, thinking of ways to destroy society to the small to large actions, he also secretly idolized the former villain Smartz, but his attempts were stopped.

To understand how or why Omni got this obsession we actually may never know about, but what we know is that the spark from true evil appeared in him for unexplainable reasons, and that when encountering criminals or villains they either ignored him or treated him like a friend, regardless of mental state or species.

Omni was also applying to Union Academy when their reputation hit, and he still went through. The application never made it as Union was destroyed, so Omni was forced to attend Unity, the replacement for Union made by the heroes.

There he was treated nicely, but was extremely self isolating. You see at that time he was aware of his goals and picked up remnant pieces of corruption at Union and was cultivating it inside his soul. He took extreme measures to hide this from the population, as Union became the face of hatred. He knew he would die if caught, and so he did the unthinkable.

We soon withdrew from Unity, but before that was said he was "curious" about the foundations of the building (Context: He had a minor in structure design and engineering) and told him that it was a simple anchor to bedrock design, and so he hatched his plan and first big thing as a villain.

Omni then managed to get/steal self boring blasting explosive and some construction suits and helmets. He then proceeded to get into contact with what remained of the darknet and got an elusive villain to dig out the layers below the bedrock until the mantle. There he got contact with another person this time rigging the bottom with explosives and a fusion bomb. Then when the preparations complete he went into action.

He posed as an engineer inspecting the foundations while secretly having the self boring explosives dig into the bedrock. He then covered the holes with dirt to cover the operation.

He waited for awhile until an opportunity arose. You see Unity was holding the anniversary for the victory against Union and all the families of students and members of the True Heroes Alliance were also VIPs there. The media was doing a story inside, outside, and above the campus. This was the perfect time to strike.

With joyful glee, Omni from a hotel room he rented detonated the explosives and watched as Unity fell and fell to the bottom and a massive explosion came out of the hole that was in Unity's place.

Omni now knew that the True Heroes and public will now indirectly fear him through Union, this feeling made Omni joyful and he soon moved to a house near a forgotten branch of the city.

The news made the attack "The biggest and deadliest terror attack in history". It made Omni proud of what he did and he didn't regret it at all. 

At that point Omni became a villain, destroying symbols or infrastructure to spite the heroes and succeeded, but it was all coming to an end.