@Zardnaar .....thank you for sharing more details.
I still think you could have used the subdual option at zero HP, but given the political details, the fact he was fighting an insectile race, the fact the player set the parameters of the duel, (To The Death), electing to kill the PC, makes more sense.
If my brother showed up to a session, with an "optimized" PAM Paladin, picked a needless solo duel,(despite my no solo Duels, DM policy), disregarded de-escalation attempts
including telling him what Stat block to look up on Google, and then he
almost succeed in the duel, I would have killed him as well.
The joy in taking the piss out of my brother, would overweigh any regret at not knocking the PC unconscious.
I add this, I think it is clear you did
not Bounty DM.
You
did take the Piss out of a PC, and that is warranted, sometimes.
Sometimes, you can solo a character level equal CR creature in 5e.
One typically needs high stats and good equipment, but it is possible to solo a 160% Deadly encounter.
My overpowered Fighter at 6th level, solo'd a Drider.
I expected that the DM would slay me if I did not prevail in the duel.
It sounds like so did the player in question.
I take back, my prior misgivings, having learned more details.
Thanks for sharing, Zard!