A recent, odd source (a let's play of modded Civilization based around Metal Gear Solid) made me curious about the man. I was sort of aware of him because he's been cast as evil, and often outright demonic, in many pieces of fiction, and from the playthrough I get the 'real' reason why. So, assuming I recall correctly and it was presented semi-accurately, a young Oda was basically a semi-wastrel, but his worst 'crime' was hanging out with 'lesser people', not caring about this fact: he just liked people for who they were. Some teacher of his he respected decided to kill himself to shame Oda for this and make Oda "grow up"; Oda proceeded to 'grow up' by trying to take over Japan, and worse, doing it in the most effective way instead of the 'proper' way. The reason he's still being literally demonized centuries later is he basically told Japan's history of rigid social orders they could all go take a leap, and succeeded immensely because of it. In essence, he offended Japan's devotion to its concepts of ingroups and outgroups based on arbitrary sociological viewpoints (see: buraku) so badly they're still making him pure evil to this day.
So, how accurate is this? What's the closest we can get to the whole story?
The current scholarly consensus is that Nobunaga wasn't actually that much different from his contemporaries. You can read about some of that here
Nobunaga's reputation has changed a lot in the 400 years after his death. The bad reputation you talk about mostly stem from his very widely known act of burning temples and killing monks on Mt. Hiei. There's also the fact that in the Edo period you're basically not allowed to criticize the Tokugawa, which means the execution of the eldest son of Ieyasu by Ieyasu's orders has traditionally been pinned on Nobunaga, though recent research has raised serious doubts on this.
Also unlike Nobunaga, Hideyoshi's popular for both being peasant-born and also the "rightful predecessor" from whom Ieyasu inherited the country. So the Confucian scholars in the Edo tend to focus on Nobunaga's ruthlessness, which, to be fair, he was. So Confucian scholars has traditionally played up the ruthless side of Nobunaga and portrayed his downfall as a logical punishment from heaven. That fact has stuck, and the massacre of Mt. Hiei would probably be the one massacre your average Japanese person that happened in the Sengoku knows, even, as noted in the linked thread, he would not actually have been that much more ruthless than his contemporaries.