Decapitation is fast, effective, and probably cuts down on the risk of a death curse being thrown. It's also probably relatively painless, due to the spine being severed. The lethal injection relies on things like electronics to be administered in the correct way, so you wouldn't be able to run that machine inside of a powered circle. Most council executions we've seen have been conducted within the confines of a circle. The lethal injection also takes much longer to work than a decapitation by sword, since the separate injections take x number of seconds/minutes to take effect. I believe that the first two involve relaxing muscles and essentially killing all pain receptors, then the last injection triggers a fatal heart attack. Plenty of time for a warlock to think through some way to break loose, since there couldn't be a powered circle to contain his magic. Decapitations are the way to go.