Rashba(1:181): Question: Regarding an incident described in Bava Kama (117a). A certain man wanted to reveal the straw of another to the authorities - knowing that it would be confiscated. He appeared before Rav who ordered him not to reveal it. The man responded that he wasn’t going to listen to Rav and that he would reveal it. R’ Kahana who was sitting before Rav killed the man by ripping out his windpipe… This raises a serious question. Just because a person insisted that he was going to inform the authorities about another person’s straw he deserved to be killed? Furthermore at that point it wasn’t certain that he was actually going to do that which he threatened. So how can he be killed on a doubt - perhaps he was simply making an empty threat? A moser (informer) is like a snake and whoever wants to kill him has the right. This that you ask how it is possible to kill him when it is not certain that he will carry out his threats. If he regularly informs then his threat is considered as if he will certainly do it. Thus the case of one who repeatedly informs is governed by the rule of self-defense, “you should kill someone who comes to kill.” Thus we see that Rav Shila (Berachos 58a) did not delay killing someone who threatens to inform on him. Furthermore all those who inform – even if it just involves money – is considered a murderer. We don’t say that we should wait till he murders and only then take him to beis din to be tried and executed. However even regarding a person who doesn’t habitually inform on others, if it is obvious that he wants to inform - as was the case of Rav Kahana where the person was told not to inform and he arrogantly said that he was going to inform – it is considered that we clearly know that he will do it. Even with this degree of uncertainty - the informer is killed. Even a third party can save the intended victim by killing the moser since it is equivalent to saving a pursued person by killing the pursuer (rodef). We don’t entertain the possibility that even though he is actively pursuing the victim that perhaps he will have second thoughts and not carrying out his threats.