Just been looking into that whole timeline of 'death games' stories question, and it's not something the internet seems to have agreed upon...
Per TV Tropes (
https://tvtropes.org/pmwiki/pmwiki.php/Main/DeadlyGame), it's all more or less based on Roman Gladiators (and similar to various Aztec/Maya/Meso-American ballsports)* but the earliest versions of this in fiction apparently start with 'The Most Dangerous Game' (1924; the 1932 movie is excellent BTW) - but that's basically just rich people hunting humans for sport. Arguably Han's Tournament from Enter the Dragon (1973) is more similar, with its contestants routinely dying at the end ofeach fight, rather than just being knocked out.
The nearest I can see that is much more like
the Killing is, apparently, a Modesty Blaise strip from 1976 called 'Those about to Die', something it's easy to imagine Wagner and Grant having read. Although I think, again, that's more about peole being rounded up for a death sport, not willing volunteers on a Game Show, in the manner of your Battle/Hunger/Squid Game Royale variety.
And then of course there's Judge Dredd: You Bet Your Life' (1977, Prog 25) and 'The Game Show Show' (1982, Progs 278-9), which sees willing contestants risk their lives for money (although they're not killing each other). Wagner and Grant definitely read those strips, because they wrote 'em!
And frankly, after doing even this cursory research, I'm giving my vote to
the Killing for being super fun and, perhaps, an original story idea!
*Pat Mills might like to wave
Murderball in your faces, too
