I'd never use "explain away" intransitively (except, fairly unusually, in the different sense of "explain at length"). To me, there seems too much distance in your sentence between "explained" and "away". I think I would write it as "Tom explained away his bad treatment to Mary".
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
EyeSeeYouto convey the same idea of explaining away.I don't know of any exact synonyms, but these are somewhat related.
AlpheccaStarsrationalize