While RAW it's not an evil act to use the soul to heal yourself, not immediately freeing the soul seems like it could be an evil act depending on the soul.
Originally Posted by Niara
A creature might have been imprisoned as the result of defaulting on a deal, while another might be the victim of a night hag's curse [...]
Carrying Soul Coins. To hold a soul coin is to feel the soul bound within it — overcome with rage or fraught with despair.
The first line tells us that not all soul coins contain the souls of evil creatures (who in 5e might "deserve" to be punished). Ergo, some souls are of "good" or "neutral" beings. Are these souls constantly feeling rage/despair, and thus being tormented, while in the coin? If so, then saving the coin for Drain Life/Query means you're extending a creature's suffering for personal benefit. Seems kinda evil.

Does the "good act" of (eventually) freeing the soul balance out the "evil act" of extending their suffering? Seems like the only ethical way to use a soul coin is to immediately use up all the charges or destroy the coin.