"It is doubtful in what sense this word [DIABOLOS] should be taken, Whether we should render it DIABOLIKOS (= TOU DIABOLOU hUPOURGOS), or EPIBOULOS, (both given in Euthym.,) it will be an hAPAX LEGOMENON in the N.T. Of the two however the latter is the harsher, and less analogous to N.T. diction. Certainly in the dark act here prophesied, Judas was under the immediate instigation of and yielded himself up to Satan; and I would understand this expression as having reference to that league with and entertainment of the Evil One in his thoughts and purposes, which his ultimate possession by Satan implies. This meaning can perhaps hardly be rendered by any single word in another language. The E.V. 'a devil,' is certainly too strong; 'devilish,' would be better, but not unobjectionable. Compare hO hUIOS THS APWLEIAS, [John] ch. XVII.6" (Alford's Greek Testament, 1:697).
"Have not I chosen you twelve, and yet one of you is the devil? (Tyndale's NT)
"DIABOLOS (#1333) slanderer, devil (DJC, 171-72). Satan has made Judas his ally, a subordinate devil (Barrett). Monadic noun not requiring art. (GGBB, 249)" (The New Linguistic and Exegetical Key to the Greek New Testament, p. 198).
Wallace believes that DIABOLOS is a monadic noun in John 6:70 since there's only one devil. He prefers the rendering, "one of you is the devil" (GGBB, 265).
"EX hUMWN hEIS DIABOLOS ESTIN. Even of you one is a devil. Lucke, referring to Esth. VII.4 and VIII.1, where Haman is called hO DIABOLOS, as being 'the slanderer,' or 'the enemy,' suggests that a similar meaning may be appropriate here. But Jesus calls Peter 'Satan' and may much more call Judas 'a devil.' Besides in the present connection 'traitor' is quite as startling a word as 'devil'" (Expositor's Greek Testament, 1:761).
"He [Judas] is a devil because through him finally the devil will seek Jesus' life (cf. 13:2, 27)" (J. Ramsey Michaels, John, p. 122).
Paul Dixon wrote in January 1997 (BGreek): "I did find 6:70b to be qualitative, as did F.F. Bruce and Hendriksen (see commentaries). It does seem the Lord was not identifying Judas with the personal devil any more than he was with Peter in Mk 8:33 (Bruce). Rather, 'His devilish character appears especially from this fact that others ever so many of them, had deserted the Lord when they felt that they could not agree with him and when they rebelled against the spiritual character of his teaching, this one individual remained with him'" (Hendriksen).