Tomas and Tereza decide that it is time to euthanize Karenin. He is suffering, and neither one of them can bear to watch it any longer. Tomas decides that he doesn’t want to give the injection—he wants to wait for the vet—but when Karenin’s suffering worsens, they know they can’t wait any longer. They gently place Karenin on the couch, and Tomas cuts his fur from one leg while Tereza holds him and whispers in his ear. Karenin jerks when Tomas inserts the needle, and then his breathing increases and stops.
Again, Karenin is completely powerless, and he has nothing left to offer Tomas and Tereza. The mercy that they show him while they have absolute power over him is evidence of their innate goodness. This level of mercy, Kundera argues, can never exist if something can be gained from it.