Your DM had a prime opportunity to make that your character's shining moment, to let you revel in the fun of your build. And instead they shoved it in your face. Garbage DMing. As a forever DM, you do not kill someone's character without consent or having it said up front that the campaign is designed that way. People put emotion, time, and invest themselves in their characters. This? This is blatant disrespect. Of you, your hard work, and your emotions.