(To Kabuto) "You just don't have enough information to explain yourself yet. Your glasses, your name, being someone's child is not something that can show who really you are. But that is fine. If you're not happy on what you have now… you can just find new things to find up. …You see, I also want to find who I really am. That's why I'm collecting a bit of everything."
(To a young Kimimaro) "Maybe, just maybe, there is no purpose in life. But if you linger a while longer in this world, you might discover something of value in it, like how you discovered that flower. Or, how I discovered you one fateful night."
To Hiruzen) "I want to obtain all the techniques and gain a true understanding of everything in this world. The first one to mix blue and yellow called the new colour "green". I want to do something similar to that. If blue is the chakra, then yellow is the seal, and green is the technique… Just as there is no end to the variety of colours, there are so many thousands… tens of thousands of techniques in the world as well. But in order to obtain every possible technique and truth, it would require an eternity. Only one who understands everything after spending such time on this can be fittingly called the Ultimate Being."
(To Hiruzen) “Well. I sort of have a goal. If I were to state it in words, I like to see moving things. They’re boring when they don’t move. A windmill that is not moving can be nice from time to time, but most of the time its not even worth looking at. Now, I want to move the windmill with the wind called the Destruction of Konoha.”
(To Hiruzen) "What nonsense! In the grand scheme of time, you're nothing more than a trivial footnote in the brief history of a cluster of huts called "Konoha"! In time, the Hokage Stone Faces will crumble and be worn down to nothing…"
(To Sasuke) "I refuse… to let this happen! I am Orochimaru! I am immortal! I will not die here! Destined to discover the true meaning behind everything! Destined to take control of everything in this world!!"
Orochimaru is an existentialist in a world of idealists. He doesn't see purpose in anything, but he does find value in certain things. These things are all tied to his self obsession. He represents the fear of the unknown, the fear that we as humans may not actually have a purpose as individuals. Orochimaru has this fear, and he discovered it after witnessing the death of his parents. This is why he pursues immortality, he's running from death, running from this idea that there is no meaning in life and he shares the destiny of everyone else, everyone else he looks down on condescendingly. He mocks Hiruzen for placing value on a village which like all things, will come to an end.
His ultimate ambition is to become a perfect immortal being, but even he can't define what that is. Regardless, he sees himself as meaningless until he achieves this perfection, and thus his sole purpose becomes to constantly better himself until he reaches this point. He sees Sasuke as superior to others due to his unique abilities, he places utmost value on the physical objective aspects of life, rather than the more idealistic and emotional aspects, and thus instead of seeking to better himself as a person within society he merely seeks to obtain a stronger body and more knowledge of science, aka objective reality.
What makes Orochimaru such a timeless and incredible character is he has genuine self awareness, moreso than most real people in general, and his actions are mostly driven by his own existential fears instead of simply selfish desires. He is truly complex and very much human, except his fear has overwhelmed the other aspects of his humanity and consumed him, turning him into the terrible monster we know. I can relate to him, and anyone who has had these existential thoughts about things like is there even a God? Why do we exist? What's the point of good and evil? Why care? Orochimaru asked all these and he couldn't find an objective answer, because there is none, so it's foolish to jump to conclusions and assume there is. Instead, he tried to find the answers and achieve purpose any way he can which led him to do his forbidden experiments and manipulate children as tools and vessels, in the end he failed however, and was forced to humble himself.
Orochimaru's reasons for being "evil" go far far far beyond simply black and white, they delve into the most complicated aspects of human psychology, and the most morally blank implications of existentialist philosophy. This is why he is above and beyond any other villain in shonen manga, not that there isn't more to his character beyond what I've written, as he has layers and layers of symbolism as well, notably paralleling various religions.
but sadly many ppl won't understand it 
