Appeal to reason, and hope for the best.

You think to yourself, despite showing compassion to mortals, you can't imagine turning on what you've been for hundreds of years. You decide your best bet is to explain to the angel that you showed compassion, and you deserve to live out your days. He towers over you, giving you slight pause before you answer:

"Listen, I showed compassion for those mortals, yes. And I'm glad I did something good. But I don't want to be an angel. I hope that for my sake, you realize that because I did something good, I'm not all evil. Can't I be free to live out my existance? It's not like I ever di-"

Before you can finish your sentence, you feel a sharp pain in your stomach. You look down, realizing the angel has buried his sword deep into your gut. You look up at him slowly, your eyes filling with fire. The angel shakes his head. "You are still a demon. And you still must take souls to survive. If you do not wish to help me... then this is your fate." He lifts the blade up and opens your chest up, demon blood spilling out across the forest floor. He pulls out the blade, and your eyes fade to black; this is the last and only thing you will ever see again.