You could describe the claw attack as deep enough to cause the foe to collapse in pain, and bleeding out, falling unconciously. A wound bad enough to drop but not kill. And the attacker, satisfied with that result, walks away without digging the claws in for a last, fatal twist.
But I will also agree with what you're saying - it's weird that a character intent on subduing a foe would use a normally fatal attack to do so. I find this to be story consideration, though, and why I also agree with the others here who say that the decision to not kill maybe ought to be made and declared before the attack is resolved - so everyone at the table can share in the same story of the party attempting to take the foe alive.
But I'm also fine with deciding after the attack is resolved. The story just becomes different. In this case, the party, at the climax of the fight, chooses to spare the foe's life.