How is lethal injection "slow and painful"? Maybe a wrongly done hanging as they strangle to death, or the gas chamber, slow and painful enough for you? The injections they give you do many things. First they give you about 5000 MG of anasthetic, 150MG is standard for surgical procedures. Next, they inject a paralyzing substance to cause the muscle to relax, stopping the diaphragm and lungs, then the toxic agent is injected, which interrupts electrical signals to the heart, causing cardiac arrest. Toxic agent isn't used in all states, however.
In no way is it slow and painful. Compared to past execution methods, they should be glad to get lethal injection. There is always the options of gas chamber, firing squad, hanging, or the electric chair.