According to your reading of the scriptures, theology, or opinion;
Was the first sinner a human or an Angel, or some other creature?
You ask tough questions.
You are of your father the devil, and your will is to do your father’s desires. He was a murderer from the beginning, Jn 8:44
The beginning of what?
I don't think the Bible is clear on this. Satan (whoever, whatever exactly that is) lied to Eve before Eve sinned, so I think some being(s) must get credit for beating Adam and Eve to sin.