The system's public brand for killing another crawler — and the one Carl did not get.
The player killer skull is the mark the system hangs on a crawler who kills another crawler, and Quasar flags its absence as a genuine result. When Quan Ch stole Carl's stairwell key on the eighth floor, Carl "pretty much had to kill him, though he didn't get a player killer skull".
That clause matters going into Faction Wars. A skull follows its bearer, colours how every other crawler and every sponsor treats them, and on a floor where ten teams are hunting each other it would be a target painted on Carl's back for free. He walks onto the ninth floor without one.
A visible brand or marker applied by the system to crawlers who kill other crawlers.
The crawl's public penalty for player-killing, notable here for what it did not do to Carl.