Well, would you feel sick at the killing, would it eat you up inside? Mugabe is effectively a paranoid schizophrenic, or on that continuum, so what he actually needs is medical care. Of course he won't get it, instead, the psychopathic apparatus of warlords and various sadistic military and police will continue maintaining him in power in order to maintain climate of fear and power for themselves. So you'd have to kill all of them as well. Then, they have children, boys, who they educated to be ruthless, power-hungry warlords too. Would you continue killing? Would any of it lose you some sleep? Would you enjoy destroying them and go out planning for more or would you be a torn apart victim of circumstance? Do you normally enjoy inflicting pain on others, especially weaker people, women, animals? All that would determine whether or not you were a psychopath, regardless of whether you actually use that weapon on Mugabe or not.
Just keep in mind if you use an example from such a starkly different culture, you are getting the result from a starkly different culture. In other words, just because some horrendous behavior is practiced in a culture doesn't make it universal human behavior. As for experiments like Zimbardo's prison experiment and the torturing the people behind the wall experiment, those show a completely different human behavior phenomena and are unrelated to the OP hypothetical.
I disagree. People have proven to act pretty reliably given a certain fixed set of circumstances. This has been true throughout history. That is the very basis of propaganda and modern marketing techniques. Starkly different cultures had never changed the willingness of soldiers to kill on command given the right training. The entire basis of basic military training is just that, to instill obedience and the willingness to kill on command. It was in Shih Huang Ti's army and it is in the armies of today. Starkly different cultures has not affected the willingness of the desperate and hungry to steal in any human society.
Here's a whole book about how the Milgram experiment was flawed. With an interview with the author on NPR It turns out, in later interviews, half the people in the experiment thought it was fake. Of the other half, 60% refused to continue the attacks. So it's a subset of around 15% who kept up the torture.
Milgram was an easy reference for the OP. And besides, Milgram's experiment has been replicated in numerous other locations with similar results, some with even higher obedience rates. But basically there are an endless supply of examples of "normal" people doing horrible things for relatively meaningless motives to be found from the news crime reports and court records, or some the of uncensored reports coming out of Iraq and Afghanistan from all sides of the conflict for that matter.
We aren't talking about killing, we are talking about killing one's children. Culture makes a huge difference. As for the experiments with acting prison guards and following commands to torture, one needs to keep the sample sizes in mind as well as the lessened focus on the fact some people refused. Not everyone universally follows the leader. Soldiering has a whole different set of variables. I see @Jack Asher pointed out there were refusers in the torture experiment, as well as people who knew the screaming was fake.
No, we are talking about killing a monster capable and willing to commit mass homicide who *used* to be someone's child. That was the OP's stipulation. To take a real life example, if the parent of one of the Columbine killers, happened to be present and carrying a gun, are you saying that the probability is that the parent (any parent) would just stand there and watch them massacre the other children in the school when they could have stopped it? If so, then I know who the real monsters are.
To some of us, our kids are never going to be 'used to be'. A parent might at least try a non-lethal shot or other interventions. Another option is self sacrifice, get between the targets and the shooter.
Depends on the person and their beliefs/values. There is no concrete answer here. If my bro or sister did something like that, I hate to say it, but I'd probably look the other way so long as it didn't hurt me, my family or anyone I loved. As long as they didn't make me a knowing part of it(In which case I would fight it), I would deny it until the end.
Er, sorry for the history-nerd intrusion here, but I feel the need to the point out that Lizzie's parents didn't see it coming. It wasn't like Lizzie burst into their bedroom one night as they were both going to bed, showed them the axe and said, "You have pushed me around long enough!" She snuck up on them when they were distracted (Abby was cleaning up the guest bedroom) or sleeping (Andrew was napping on the couch) and by the time they saw her, it was already too late. Though from what I know of Andrew and Abby Borden back when I was reading up on the 1892 murders, had they seen her coming and had the time, they likely would've either fought back physically or tried to talk her down or something. They would've done something had they known what was about to happen. But would it have ever occurred to them? Of course not! They didn't raise her to go around axing people to death. They raised her to be a nice, polite lady, so I imagine that had they realized exactly what she was about to do, and had time (ie, not the mere fraction of a second they had), they would have understandably been horrified and enraged that their daughter would even consider this act.
Actually, there's some evidence that a surprisingly large number of soldiers don't kill on command, despite their training. I Googled research soldiers don't kill in war and found some links. "Hope on the Battlefield" (berkeley.edu) talks about how easy it is for a soldier to deliberately fire high and therefore not actually fire at the enemy, and discusses evidence, based on kill rates, that soldiers have historically avoided shooting at the enemy. It does, yes, go on to discuss the measures taken to change modern soldiers so that they are willing to fire, but the general vibe, all the same, is that it's not the least bit easy to make a person kill another person. On the original question, I think that it depends enormously on the details. Would killing the person save other people? Is there no one else prepared to stop him from killing other people? Would the person that I knew, before they changed, want to be stopped? If they could know for an absolute certainty what they were going to become, would they have killed themselves? And I suspect that child versus sibling would be different, and older versus younger sibling would be different.
Actually... Honestly? This happens everyday. It's called military snipers. They - and their spotters - actually see the people they kill one by one. They sometimes can even see the faces clearly and watch their victims die. Then, after a three or four month rotation, they are supposed to go back to a 'normal' life and pretend they didn't spend the last quarter of a year killing people one at a time. The offshoot of this is a very specific form of PTSD filled with a lot of anger and, oftentimes a bit of guilt. But this is mitigated by the awareness that, while taking one life, they are saving thousands of innocent lives. Unfortunately, too often, that PTSD can get the better of them and it can, in some cases, destroy them physically as well as emotionally and psychologically. Actually, ANY person is capable of such evil given the right catalytic set of circumstances.
History-nerd intrusion, Part II: Chances are, Lizzie didn't even kill her parents (actually her father and stepmother). Poor , maligned, Elizabeth. Prosecution had such a flimsy case, with highly suspect police reports and weak witness testimony, the Defense case was actually won on the Prosecution's witnesses. And, in the end, Lizzie was completely exonerated.
Now, as to the question of parents willing to kill their own child. In the wake of so many horrid mass murders lately, I have seen three parents who have expressed sorrow and regrets to the victims. Most recently, a mother was quoted as saying, "My pain is for the victims, not my son." Is it too far a stretch to believe a parent could then kill his or her own child? And then, too, I can recall more than one parent who has killed their own young child/ren. It's not really too far fetched to believe, under stress, they could willfully kill their adult child, too.