That was the demon god dood's sword. And he didn't put it there until after the guy had escaped them.
Take your pick, or any combination.
1. They were stunned/surprised/flat-footed he betrayed them and didn't move to stop him or
2. They lost track of the Demon God and didn't want to risk drawing attention with fighting Rufus OR
3. They didn't want to attack another human because they're either not used to or conditioned against it OR
4. Jade was exhausted from using his skill the Beastmaster repeatedly cause asshole Rufus kept drawing aggro from the previous chapter OR
5. Jade's shield was cracked from tanking the hit fromt he demon god and he wanted to conserve strength than fight Rufus.
And yes I know it was the Demon God's sword, no he didn't just "put" it there, he speared Rufus by throwing his sword. Your comment sounded like you didn't even realize Rufus was killed.