He considers letting Johnny go, letting this be retaliation enough for the mark. He also considers letting him go only so he can catch him again, what a surge of hope and then despair that would be for Johnny, swimming for the light of the stars only to be pulled back down. But he opts against it in the end, generously, not the least of which because the water isn't all that deep--they're nearing the bottom even now, despite Johnny's struggles surfaceward. It's possible that he can make death a permanent state of affairs in a dream, a gift to take back to the waking world. Zagreus doubts it, of course, but perhaps this is as good a time as any to find out. Zagreus doesn't loosen his various grips no matter how Johnny writhes, until he's sure it won't be any use, that Johnny doesn't have strength or oxygen left to struggle away even if he does break free. He wants just enough distance to be able to pay attention, holding Johnny securely but at arm's length, mouth a firm unbreathing line and eyes avidly bright. What a rare treat, to murder someone and have them survive to remember it unfondly.