Because they did in the worst way possible. All Alabama had to do was flood a sealed room with nitrogen and the execution would have been fairly “unremarkable”. Instead they forced a has mask on Smith that required his cooperation to function properly, didn’t have a one-way valve to remove exhaled gas, causing CO2 to build up in the tiny mask.
A haircut is also a painless and quick procedure, but that doesn’t mean your barber can’t be incompetent and totally fuck up your scalp.