It won’t be painless death, not to say he deserves one. Breathing nitrogen will be no different than breathing any other inert gas like , say CO2. Think breathing into a plastic bag. It will hurt for a while until he passes out.
In the absence of oxygen he will take deeper and deeper breaths, in gasps. The heart will start pounding trying to get more blood to his brain. Blood pressure will shoot up. Eventually he will suffocate to death. This is no way a painless death. Sodium Thiopental (one drug injection) is probably the most humane. Puts you to sleep and then you die in your sleep.