"A man may do as he pleases..." It is a simple fact that just about anyone can get a gun and shoot another, or a knife and stab him. What happened in Seadragon(assuming it was not suicide, which seems a fair assumption) is the proof of that statement. The killer did just as pleased him to do, which was to kill the farang. Unfortunately, God gave us this power and this freedom. Why, I don't know. Perhaps to test us.
There are a lot more pychopaths in this world than you might imagine, although I think sociopath is a more accurate term for what we are talking about. Most of them do not kill other people only because they are smart enough to understand the risks and fear the penalties. Those are called functional sociopaths. In their minds, they are justified in doing anything that they think is right, and they have no sympathy for anyone else.
But there are a whole lot more people, who are not sociopaths or pychopaths, except for one moment when they lose control of themselves. Then they become psychopathic until they come back to their sensed. Clinically, they have a psychotic episode. Usually, these episodes are referred to as a crime of passion, or temporary insanity. And, in certain cultures, such episodes were considered reasonable reactions to circumstances, and not prosecutable as homicide, such as when a man found his wife in bed with another man.
So, if you look at my words, you will notice that I did not ever say that it is acceptable to kill. Morality is certainly a good thing in that it proscribes such useless, and destructive behavior. Bhuddism, I am quite sure, proscribes acts of murder and mayhem. But, unfortunately, men often, be they farang or Thai or whatever, often put personal pride before morality. Why I say it isn't a matter of right or wrong, is that in such instances, I don't think people stop to consider whether they are right or wrong. They just feel the need to do what they do, even if it is wrong. I think we all agree that it is wrong(at least I hope so), and I think that most, if not all Thais, would agree also. In the end, it is what it is, it is simply a fact of life, right or wrong, like it or not.