Are both of the sentences correct? If they are, then what is the difference in meaning? No article is used with evil. Is it okay?
1: This thing by itself is not evil.
2: This thing in itself is not evil.
A thing by itself is alone, unaccompanied by some other thing.
A thing in itself is a thing considered only from the viewpoint of its being what it is, not from the viewpoint of what someone might do with it.
Without an article, evil is simply an adjective. There's nothing wrong with this.

This thing in itself is not evil.

Here's a simple example of how you might use this phrase.

A gun in itself is not evil, but it can be used by an evil man to do evil things.

Best wishes, Clive
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