strike wolf wrote:It's easier said than done. You may not be able to live with yourself afterwards but fear can be a powerful influene in deciding whether or not to act.
This is true, but it does not make it right.
However, the real answer depends upon a lot of details not given here. Was the guy already dead, for one? Was there a way to grab him without endangering anyone else nearby? Etc.? Also, not to be gruesome, but anybody strong enough to actually decapitate another person with a knife is probably "crazed", either from drugs or just mental illness. It would take more than just one person to subdue them.
In my case, a very BIG issue would be my kids. If I had my toddler or even older son with me, I would get them out. For that matter, I would worry about other kids, particularly any not right with their parents, before I worried about the adult victim.
Basically, if I had a chance to help the victim and not escalating the situation or such, I would. However, most likely, I would not have that chance.