Allah the Almighty has created Satan not to seduce, but to test mankind. Yet Allah did not create any shaitan first, the shaitan chose to be a shaitan and Allah allows him to test us. When you say,"telling truth is good", why? Because telling a lie is bad. If there is no bad thing, good thing can not be compared, because we can say anything good, when any opposite thing like "bad thing" is existed. And such good and bad are here in this world so that Almighty God may test us which of us are good in deeds. "He Who created Death and Life, that He may try which of you is best in deed: and He is the Exalted in Might, Oft-Forgiving" (Al-Quran, 67:02) The arabic "Al-Shaitan" means "the one who transgresses the limit"(like the one who does injustice, does wrong-doing, does not obey the commandments of Almighty God, whispers bad things in one's mind etc). Here jinn and human both can be satan. Quran says, "[The same] who(evil) ...