“Saint” or “Sancte” means “holy.” The Angels who remained faithful to God are holy. According to scripture. Michael was the Angel who led the two-thirds of the angels who remained faithful to God. The remainder rejected God thus lost all their beauty and goodness that come form God’s life in them, thus they became devils.St Michael is truly holy, therefore there is no problem with calling “him” “saint”.
We also use Sancte in relation to the Holy Spirit. who also isn’t human.
I’m not sure if it’s part of Sacred Tradition… or simply a pious story; but I had also heard, that before the fall “Lucifer” was the most beautiful of all the angels. But because of his disobedience and disloyalty, to God… and his rebellion… he became ugly.
I always thought, that the ugliness of Satan and the fallen angels… lay in their sin. In other words, their true appearance… which God surely sees… is extremely ugly. So, although the devil can appear as “an angel of light” and deceive human eyes… he can never deceive Almighty God.
(btw… Nor can we. God sees the true condition of each human soul, as well).