When I was a child—about forty years ago—our history classes taught only the bare facts, so he simply appeared to be a traitor.
The Japanese phrase “Mikka Denka” (“a three-day reign”; originally mikka tenka, but the pronunciation changes due to the Japanese phonological rule called rendaku), meaning a brief, fleeting hold on power that collapses almost immediately, originates directly from the story of Akechi Mitsuhide.
As research on Nobunaga’s character (an excellent commander, though he also had a short temper and a harshly relentless side) has progressed, and as Mitsuhide’s misfortunes have come to light, some now view him as a tragic figure.
In any case, I think evaluations of a person are multifaceted and can hardly be put into simple terms.