The way Naruto generally portrayed good and evil originally was the same as you see it in every other cartoon / anime / manga. Naruto is introduced as a hero and as a result his enemies, in this case Orochimaru, is played out as the evil character. As the series progresses and we start getting deeper into Shippuden, we begin to see a different side to the evil characters. But as saadpanda already stated, evil is merely perspective. What evil means to one person can be completely different to what it means to another. This same concept can be used in relation to beauty. You may think somebody is attractive, and because of this, think that makes them beautiful. Whereas, another person could find a persons personality kind and find that beautiful. A more basic answer to your question would be that Orochimaru was represented as evil because that's the way Kishimoto wanted you to see him.
