forgive your friend...and make an exit in a graceful manner . delete/ erase/ change your circle of friends but remain civilized to your liar friends... don't stay close anymore not even to trust again because a liar friend will only put you into circle.it is the attitude / trust/ honesty that you make bond to a friend that counts a lot. if you forgive and stay close to the liar friends the tendency or mistakes will be repeated.thats the fact of life. have a lesson to remember.be reminded of your experience (s) with that liar friend. Goodluck!

Kill him

It depends on their reasoning for lying.  The situation is really the determining factor here.  There might have been a logical reason for lying. Such as being in a situation where you could get yourself and others harmed or killed as the result of you telling the truth.  Otherwise, you really shouldn't lie.

It's all about the motives behind it.

At first, i'll ask him or her why lie to me. If the reason is acceptable, i'll understand him or her.

Even you watn to trust your friends,  you had better see  what extent of trust you may render . Do not trust your friends without any limitation .Be cautious to your option and trust and I think you will really trust your friends .