Earlier quoted context omitted.
> To deceive someone is to tell them an untruth for personal gain. No, it's to cause someone to believe something that is not true. Personal gain is a common goal served by deceit, not part of it's essential character.
Causing someone to believe something that is not true is too weak of a definition for deceive. Consider the difference between telling someone a falsehood versus deceiving someone. There is a malicious intent behind deception, which is not present in falsehood.
No, it's literally the definition of the word.
> Consider the difference between telling someone a falsehood versus deceiving someone.
The difference is intent; if you don't know that it's false, or you don't intend it to be believed, it's not deception.
> There is a malicious intent behind deception,
It certainly is a matter of intent, but aside from the sense in which a false belief itself is a harm, and any intentional element of harm even if not a net harm is “malice”, I wouldn't say it is necessary malicious.