For the sake of discussion, let's forget about logical inconsistensies like paradox of free will, omnipotence paradox, theodicy and other things that might hinder a pleasant analysis about most monotheistic religions.
Why is Satan evil?
This graph demonstrates the death tolls of both of the parties. Only events with a specific number were counted so it doesn't include the flood, plagues, famine, Sodoma nor Gomorrah.
The ten people killed were Job's family and God gave his permission to this action.
I also found this to be a compact package.
In comparison Satan looks like a hero.•Satan, like Prometheus, gave knowledge to humanity by giving Eve the fruit from the forbidden tree. Because of Satan, humanity gained knowledge of good and evil, according to Genesis. Since we couldn't have possessed knowledge of good and evil before eating the fruit, Adam and Eve couldn't have known that eating the fruit was evil, so it seems a little harsh to punish them as severely as God did. Satan gave humans true capacity for moral judgment, unlike God, who simply expected everyone to mindlessly obey his orders.
•There is no biblical record of Satan engaging in the murder of torture of any human being, unlike God, who is guilty (and proudly guilty) of commiting genocide.
•There is no biblical record of Satan ever ordering someone to kill someone else, unlike God, who has repeatedly demanded the deaths of those who commit even the smallest of offenses.
•Satan will not be holding a massively dramatic ceremony full of blood and death for the return of his son to Earth. God apparently will.