Translation:He is running away.
The "s" (or "se" for verbs not starting with a vowel) is a reflexive pronoun. It reflects the subject of the verb and is more common in French than in English. In English we may say: "He sees himself in the mirror" but not "he runs himself away" as it would be the case in French.
just because i've been driving myself mad too...for adult language students, we tend to compare new languages with our own, causing more confusion than necessary. All i would say my friend, is to simple learn and accept it. Not everything translates and not comparing our own language to that which we are learning is half the battle. I feel your pain.
Not always, it could be sometimes possible. You could try reporting it. https://dictionary.reverso.net/english-french/He%20runs%20off
Here is a verb conjugator for you and it comes with a dictionary: http://conjugator.reverso.net/conjugation-french-verb-s'enfuir.html
"s'enfuir" is a reflexive verb; this means that the subjects refers back to itself. Literally, "il s'enfuit" translates to "he runs away (himself)", as you pointed out. However, it is always a bad idea to translate sentences literally. You would never say "he runs away himself" in English, would you? No. So, think of "s'enfuir" as a regular verb, but with an 'invisible' reflexive pronoun. It's there, just don't translate it into English.
Yes, you would use "fuir" when you run away from someone else. "enfuir" is always reflexive. https://dictionary.reverso.net/french-english/S'enfuir https://dictionary.reverso.net/french-english/fuir