1/4 of jesus ministry was preaching on hell casting out devils himself and demonstrating that there is an enemy that needs to be dealt with and as i said he was living in old testament times and to the jews of the day this behaviour was not alien.
to say that the jews did not understand that there is an adversary or an opposition is displaying a lack of understanding of what they believed. but as in jewish so in gentile circles - some believe others dont
if satan is merely a figment of mans imagination then man is more evil than one likes to admit however man being what he is, he doesnt really need much urging from gods arch enemy
other jewish sources also point to this character such as the book of enoch, the book of wisdom etc. pieces and scrolls of which were discovered at Qumran
another point to make is that it has become fashion to call many by one name and it is usually the name of satan that is attributed to others - again see the book of enoch. these books btw were quoted by peter and others in the new testament.
the closest we get in the ot that would describe such a personage as satan would be isiah 14:12 and ezekiel 28:2ff - the king of tyre may well have been a man but the attributes described in the next few verses such as "anointed cherub" could not be applied to a man but to a fallen angel. the anointed cherub is usually described as being satan or possibly more correctly lucifer (helel, the light bearer) of is 14
