Execution by plastic explosive.
Dig a deep trench.
Put a crude wooden chair made of railroad ties at the bottom.
Wrap a 5 pound mass of C4 around a football helmet, in 5 separately fused & wired sections. Hook it all to 5 different power sources, each capable of detonating all 5 blocks.
One switch.
Multiply redundant charges, with separate power sources, make failure unlikely.
Instant, utterly painless, & completely certain.
Then, fill in the trench.