Lyxen
Great Old One
Your DM should instantly rule your character is Chaotic Evil, burning defencless souls into agonize destruction is worse then murder, rape, torture of flesh, thief, etc, and there simple us no relationization that makes something so heinous acceptable or even morally neutral. Even evil souls don't deserve that and many damned souls weren't evil.
Sorry, but no. All the souls sent to Hell are evil. After that, maybe some non-evil souls got captured in Soul Coins, but it's more likely people who had made a deal with a devil and tried to renege on it. And is it better to destroy them, or to leave them around to suffer imprisonment and torture for all eternity, or worse to become devils to perpetuate the cycle ? It's certainly not a good act, and it's very probably evil, but using evil to destroy evil is fair. Also as a reminder: "few people are perfectly and consistently faithful to the precepts of their alignment." So an immediate shift to evil would be a bit strong for me.