In Pali the prefix a- can mean “opposite of…” (paṭikkhepa) as well as “absence of…” / “remoteness from” (virahita).
In the case of abyāpāda and avihiṃsā the commentaries understand both senses of a- as possible but more often take it in the paṭikkhepa sense, making the two words synonymous with mettā and karuṇā.