He is portrayed so because many Japanese samurai commentators regarded him as upstart usurper that destroyed harmony of Japanese idealized samurai hierarchy. In their view, he is the motorcycle gang leader complete with punk hair and bad attitude. He is ruthless, brilliant, unorthodox and uncharacteristically non conformist hence his constant portrayal as dark lord from another dimension.
Confucian influenced samurai commentators did not like upstarts hence they often written Nobunaga in rather bad light, only concede grudging respect when it warrants so such as his conduct in Battle of Nagashino 1575 CE (the battle that signified the end of famous Takeda clan) and his putting down of Ikko-Ikki monk rebellion. Not to mention his ruthless campaign against Iga ninja clan (as shown in GONZO anime series Basilisk) which some historian compare it to Mongolian genocide against feared Assassin clan during the Crusades. This is the man who hired an African as his personal bodyguard, he purchased him from visiting Portuguese missionary which brought him as slave. He is also the one promoted sandal bearer to general which later inherit his legacy and finish uniting Japan in his wake. The sandal bearer's name is Toyotomi Hideyoshi whom Nobunaga nicknamed him "Saru" which means monkey. This is why Hideyoshi was carrying a pet monkey in Sengoku Basara series.
However the one that upset so many samurai commentators is his liberal usage of gun on Japanese battlefield, they considered gun has changed the nobility of samurai warfare. The katana blade and bow is no longer elegant weapon of decision which celebrated in timeless samurai poetry, it is a simple peasant with arquebus can gun down even the mightiest warlord from safe distance riled samurai purists to no end. Hence it is no surprise that they look on Nobunaga with sullen disdain for this tradition breaking aspect.
In latest game inspired anime series Sengoku Basara / 戦国BASARA, it is an easy template for story writers to copy this popular casting of Nobunaga as badass baddie instead of more understanding examination of him. Just like how most Romance of 3 Kingdom media usage of Cao Cao as villain most of the time.
The series is fun to watch but don't take it too seriously. However the cast VA is like who's who in Japanese VA industry which is like Yang opposite of concurrent infamous Queen Blade which boasted very impressive female VA lineup Ying (the anime with lactating acid breast ahem). Nobunaga is voiced by famous Norio Wakamoto which considered by many fans as most macho male voice comparable to James Earl Jones Darth Vader.
It is basically a Gurren Lagaan with samurai setting with little homoerotic spiced relationships (Yukimura and Lord Takeda is perfect example).
Somewhat sufficient Wiki entry
