Sauron was created by Eru Iluvatar to desire order. He saw war, infighting, etc. across Middle-earth/Arda. He saw a world full of chaos, and inefficiency. He joined Melkor of his own volition, because he saw Melkor's methods as more efficient. Then, over time his methods became more extreme because of push back from the Valar and Maia.
Is Sauron truly evil, if he was created to crave order and efficiency, and simply became corrupted by his own obsession? Or, is it just that: He's a corruption.
In the case of Melkor. Melkor simply wanted his own will. He fought against what was designed for him, so he could have free will. He fights against Eru Illuvatar's creation, seeing it as a battle for free will. They're both forces of nature. Forces of nature, and corruptions against Eru Illuvatar's creation/plans, but I'd argue, not truly evil. Extreme methods, for sure.
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Furthermore, there's a truly fascinating dichotomy of power here, because Sauron joins Melkor because Melkor's methods were more efficient, as Melkor was creating races and peoples with free will, that is to say, he was creating races not inherently subservient to Melkor, how can one truly be evil if this is their objective? Their methods became more extreme, leading to their capture by the Vala, but they were just fighting for their own will, and a free world. Then the dynamic of Sauron obsessing over order, and freedom in many ways is the cause of that disorder. So he wants to take freedoms away, but not initially. He only came to this conclusion much later, as he found it to be the most efficient method.
Sauron doesn't take by force. He uses deception, but when he got other human kingdoms on his side. He didn't trick them either. He used bargaining/trade and negotiation. Diplomacy. As he probably found these methods more efficient than using brute force. He is cunning, and did use deception with the Rings of Power, which earned him The Deceiver moniker. None of this seems to paint Melkor or Sauron as evil.
Fighting for free will and fighting for order and efficiency. Damn, what monsters. I mean, as I said, they both become extremists, in Sauron's case, because of his very own nature, giving him this obsession. But still, Eru Illuvatar is at fault for giving Sauron the desire for order and efficiency if anything.
I'd also like to point out how if Illuvatar is to be believed, ALL of this was according to his Grand Design, "And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined."
My theory, is that the reason we see Sauron as evil in The Lord of the Rings, is because we're following it from the perspective of Gandalf, Aragorn, Boromir, Frodo, Sam etc. So to them, Sauron is evil.