A "simple" concept, yet fresh and new, as well as fun and full of potential, I love it!
I love the idea of using Quake for something completely different, yet fun and I think this is a promising mod, Plumb! It's cool to watch the fiends fight as well as the shambler giving away rewards.
The only glitch I experienced was winning the first round, then losing the second one and my health going down to 75 instead of 100 (it had gone up to 125 first). Apart from that, it was fine. Maybe the winner could die a little faster once the fight is over, but that's just personnal preference.
Maybe it's just me, but I had a hard time telling the difference between fiends. Maybe a more distinct skin would help, as I would have liked to be able to see more clearly which was the one I had put a bet on when I watched them fight
Obviously, variations around this mini-mod could turn it into a full-scale mod on its own right. Add more characters, arenas, maybe some hazards, special powers, enabling fights between species... etc. A special deathmatch version in which players would be able to "upgrade" characters with the money they've earned would be fun, too (just an idea).
Good stuff!
Note: Pest Control also sounds fun!