To be perfectly fair, most people have the imminently mature response to a knife in the back of attempting to plant said knife in one or more eyes of the person who gave it to them. This is rather true even of modern humans; it's called "self defense" in American courts of law (or, at least, the attempt to do so out of belief that failure to do so will result in near-immediate death is). In more rough-and-tumble times, such as most commonly are depicted in D&D, hostile action is often met with hostile action, and betraying a trust (guestright, hospitality, alliance/friendship) is considered worthy of distproportionate retribution in no small part due to the damage that gaining and then violating trust enables even those much weaker than their victims to do.

Which is all to say that it's perfectly understandable that retailiation will occur. This normally prevents a lot of that kind of betrayal in situations attempting to model real human behavior. The separation of player from character and the fact that many players are trying with varying degrees of sincerity and success to put themselves into the mindsets of fictional people can often mean that intra-party conflict arises and escalates to using mechanics against each other. Remind people that the actions of a PC should not necessarily reflect the desires of a player, so not to take things too personally, but also do not prevent PCs from self-policing within the group. You can ask the table if they're enjoying that kind of confict as it arises; if they are, let it go. If they're not, don't permit it. While taking control of a player's character is a major no-no, telling them they are not allowed to take PvP actions is reasonable. (Just be careful, because some players will start looking for excuses to do it "innocently," then. You can't always naysay it, but be watching for a pattern.)