Satan, meaning "the adversary" was mistranslated into Greek as
Diabolos, which means "the accuser" which got mistranslated into "the slanderer" and so Satan gets a bad reputation as an inherently evil character. Satan is God's right hand man, so to speak, testing the faith of mankind at God's request.
Mankind is not too keen on this and so makes him out as being evil.
Like the taxman, he is only doing his Job