Once again there is some debate as to how we are to understand this phrase. It could be that the false teachers speak disrespectfully of Satan and evil angels when holy angels who are higher in strength and power refuse to do so. However, given the fact that Peter does not refer to the example of Michael and Satan (see Jude 1:8–10) and ties the contrasting behaviour of the angels more closely to the behaviour of the heretics, it is unlikely that he is referring to fallen, evil angels.1
Most probably, the ones whom the angels do not slander are the teachers whom Peter is condemning. Where the NIV translates this verse as angels bringing slanderous accusations against celestial beings, other translations (e.g., ESV, NASB) are closer to the original Greek text which simply refers to them.
Angels are thus not slandering other angels. In fact, angels—who would have far more right to accuse the heretics than the heretics have to accuse them—do not bring against the teachers a charge that the Lord has spoken.2 They leave judgment to the Lord. The insult which they received from the heretics is not avenged.3 In this the angels’ practice is consistent with that of Michael (see Jude 1:8–10).
11 whereas angels, though greater in might and power, do not pronounce a blasphemous judgment against them before the Lord.