Still an animal doing instinctual things, orcas aren't evil and they're widely believed to be extremely intelligent. They attack ships to protect their territory and murder things to use them as play toys but that is just what they're programmed to do.

Good and evil is a moral distinction, a dragons morals aren't likely to be identical to yours or mine.

He behaves like any other greater dragon in hoarding wealth and defending it with great violence, that doesn't make him evil. Truly in a world where dragons exist holding gold is also a choice to take the risk of being attacked for it by dragons. It's a direct human analogy but they made the instinct to be hoarding rather than an outcome of capitalism as humanity claims it to be. Which leaves the question is humanity instinctually evil? So if your answer is smaug is evil humanity is evil if your answer is nature isn't good or evil nature is just nature then humanity isn't evil and neither is smaug.