They failed specifically because they didn’t try to fill the room with nitrogen. The dude was insistent that a priest be allowed to stay in physical contact with him for the duration of his execution. So they couldn’t fill the entire room, because the priest was in the room with him.
Instead, they tried using a gas mask so only he would suffocate. The issue was that the mask failed (or wasn’t designed properly, or wasn’t used properly,) and oxygen was allowed to get into the mask for an extended period of time; His attempted execution lasted for way longer than it should have, because he was still able to get oxygen.
They failed specifically because they didn’t try to fill the room with nitrogen. The dude was insistent that a priest be allowed to stay in physical contact with him for the duration of his execution. So they couldn’t fill the entire room, because the priest was in the room with him.
Instead, they tried using a gas mask so only he would suffocate. The issue was that the mask failed (or wasn’t designed properly, or wasn’t used properly,) and oxygen was allowed to get into the mask for an extended period of time; His attempted execution lasted for way longer than it should have, because he was still able to get oxygen.