Angel vs Devil

John 6:70 70 Jesus answered them, "Did I not choose you, the Twelve? And yet one of you is a devil."

Angel - means messenger of God - i.e. one who like the Sent One, communicates everything who the Father is and what He says to others

Devil - means slanderer - one who warps the nature of God to others, that he may be worshipped

check out this attempt on the 'Slander of Judas':
http://www.goingtojesus.com/topic_satan.html?tname=tfe02-16

- click on "Judas the Slanderer" parts 3-6