Iblis refuse to bow down to Adam and thereby became the first disbeliever.

We said unto the angels: Prostrate yourselves before Adam, they fell prostrate, all save Iblis. He demurred through pride, and so became a disbeliever. Quran 2:34

Iblis said that because he was created from fire, he was better than Adam who was created from mud.

(Iblis) said: I am better than him. Thou createdst me of fire while him Thou didst create of mud. 7:12

For this he was expelled from Paradise and forced to roam the earth until Judgement Day tempting humans (except perfect Muslims) to do evil.

He [Allah] said: Then go thou forth from hence, for lo! thou art outcast. 15:34

He [Iblis] said: My Lord! Because Thou hast sent me astray, I verily shall adorn the path of error for them in the earth, and shall mislead them every one, Save such of them as are Thy perfectly devoted slaves. 15:39-40