Daniella, you mustn't waste time on sox, and Freyja and shit.
If there were no man made laws, and we had to fend for ourselves, your proclaimed gut responses would be correct. But again, the environment would have to have no written and upheld rules or laws that are against killing your own species.
Even so you'd have to realize, your own tribe, or clan, or village what have you, would be very uneasy about what you've done, In that case killing your deceiver would still come at a price as you could be exiled or worse.
If killing your deceiver was possible to get away with, you'd best console in your tribe before punishment can commence. They might still decide that the both of you should be destroyed.
Ultimately whatever you do will determine your intelligence. Some will fall and you can be just another one that does just that, over an impulse. Not even the most flexible "natures law" would reward stupidity.