Inflicting unnecessary pain on even the most heinous villain is, in the end, still cruel and inhumane. Death takes care of the bad guy; but to purposely cause pain by such methods is to lower oneself to the most base levels. It's just not right. Kill 'em; but kill 'em quickly and spare them, and yourself ,who has to live with your actions, unnecessary suffering.
If all that was true, we would still use the guillotine. Even lethal injection has problems with killing someone "humanely". I could think of several methods that would do the job quickly and without pain, but people would still call it inhumane. My point is that people complain that these criminals suffered suffocation vs breaking of the neck. I'm not advocating to cause them more pain, I just don't really care if a mishap occurs and the criminal suffers. I save my empathy for the family members of the criminal's victims. Though they know that the executed will not harm another person, their suffering will not end nor will it be eased.
I guess my empathy meter is low, working a customer service type of job for over 20 years will do that to you.