Beyond Oscar
Compass Carburettor arrived before Runka, having already absorbed Fowma's Invoked Spirit, staring at the unconscious figure before him. He didn't attack the other party though, for that wasn't his intention.
It was impossible to absorb Mental fragments from someone's Dreamland. If Runka dies now, only the Mental Fragments of his body would be available to absorb. As for his Invoked Spirit, it would vanish.
The only means of obtaining it too was to trap the Invoked Spirit in the mist. And the only way to do it was to kill him the moment he summoned it while ensuring his Invoked Spirit was trapped within the mist.
This was pretty tricky, a reason he had been biding for time all along. After all, even if he trapped Runka's Invoked Spirit in his mist before, the moment he dies, his Invoked Spirit would start deteriorating, despite being trapped in the mist.