A firing squad is a quicker, more humane death than the injections. A near instant death with perhaps a few seconds of pain at most? The gilloutine although medieval and bloody too is also humane for the executed in quick fashion. Injections, electric chair , gas chamber not so much.
The most humane way to execute someone would be by nitrogen hypoxia. Several states have put it on the list for use and I believe Oklahoma has it as their primary form of execution once the protocols are worked out.