I do not think that buddhist monks actually assasinated a king, although I did not read Tibetan history. Perhaps you have mistaken someone for being a monk.
http://www.kungfubc.com/history.htm
Here is a quote from the website: "No Buddhist monk would ever carry a weapon or any other tool of war which could cause harm to any living being. By doing so, a monk would be in direct contradiction and violation of his oath, morals and Buddhist faith."
I am not very sure of Tibetan history, but here are words clear and direct from the Buddha, with clear and direct purpose - to clear any doubts about how serious Buddha sees killing as, written in the Brahma Net Sutra:
http://www.cfr.org/pdf/correspondence/xFaure.php
"If a child of Buddha himself kills, or goads someone else to kill, or provides with or suggests means for killing, or praises the act of killing or, on seeing someone commit the act, expresses approval for what that person has done, or kills by way of incantations, or is the cause, occasion, means, or instrument of the act of inducing a death, he will be shut out of the community."
In Buddhism, even a "suggestion" that has a potential to lead to a real killing, is not allowed.
It is clear that
no killings done by a monk, will be acceptable in Buddhism. I believe monks who kill for whatsoever reason will be kicked out of the Sangha community, but as far as I have known, there has been no such cases, although I cannot be sure about that.
Yes I agree that all ordinary human beings are fallible - but buddhist practice, if practiced correctly, will lessen the potential by a lot. Buddha is one example who is no longer fallible to violence. Instead of resorting to violence, Buddha will radiate Metta (love) instead, as Buddha's saying goes, "the only way to stop hatred is through love".
http://www.studentgroups.ucla.edu/buddhist/library/metta/metta8.html
There is a story about a drunk elephant sent by his evil cousin who actually attempted 4 times to assasinate Buddha himself, only to fail, and the elephant charged at Buddha, but through Buddha's practice of Love (metta), the elephant stopped in front of Buddha and bowed down. Of course not all buddhists will be able to do that, but buddhists believe that everyone has an equal potential to become a Buddha, everyone's buddha-nature (Awareness) is full and complete.