The previous answers here tended to focus on Sino-Tibetan relations and dispelling the notion that Tibet was a wonderful land of peace and serenity by citing examples of violence and war perpetrated in Tibetan history. I won't rehash Sino-Tibetan relations, see my answer here or the Works Cited in the bottom. None of the previous posters were strictly wrong, however. I'll be the first to cite examples of politicking, assasination, native dictatorship, statecraft, warfare, etc. in Tibetan history.

That said, the Tibetan tradition of peace and non-violence does have a basis in Tibetan history. The Dalai Lama likes to cite that Buddhism "disarmed" Tibet, making it a realm unable to defend itself from attack. This isn't... "wrong" but it's a vast oversimplification and is a skewed perspective on Tibetan history. Monks are strictly forbidden from taking life and are discouraged from violent activity. Historically, the two things to do to end a Sangha (monastic community) is to marry monks to nuns and conscript them into the army. Langdarma, the last Emperor of Tibet, allegedly did this to end the Buddhist sangha, supported by Tibet's native priests. When the Communists invaded Tibet, it's recorded that they did simlar activities. In some communities the People's Liberaton Army offered to pay women if they could seduce a monk (forcing him to disrobe in shame). In other instances, Tibetan nuns were passed as sex slaves in the barrackses.

In case it wasn't clear, the only two activities that demand immediate expulsion from the Sangha are sex (coitus between a man and woman, specifically) and murder (of another human).

"Violence" in the Tibetan context is defined strictly as physical violence. For this reason, Tibetans have long been discouraged from joining the army. Often referred to as a "black job" in the same vein as butchers and meat sellers. For the same reason, assasination in Tibet historically takes the form of poison and black magic. Neither are considered "violent" in the same way as stabbing, shooting, or twisting someone's head around until their neck snaps (how Langdarma's men killed his brother the Emperor Ralpacan).

The reason why violence is looked down upon in Tibetan Buddhist context is because violence (in Buddhism) stems from anger, one of the Three Poisons, along with delusion and attachment. These Three Poisons are what produce all the suffering of a human being, and lead to the cycling of Samsara. Even if a believer doesn't necessarily care about their own personal enlightenment, appearance has a lot to do with it. For example, the Zhabdrung Ngawang Namgyal, the father of the modern state of Bhutan, took a vow of celibacy to increase his legitimacy in the eyes of the sangha and the population. When his lineage and succession were under threat, taking back his oath was under consideration, but the fledgling state couldn't afford the loss in legitimacy at such a critical time (repeated war with Tibet and their army of Qoshot Mongols) so the Zhabdrung kept his vow.

So appearance of rooting out the Poisons from one's body is important. Especially in a government filled with the alleged pious. For this reason, poison (the chemical kind) is preferred because it doesn't fit the form of violence. Meanwhile skillful means (masters using physical violence to assist in spiritual advancement) and Dob-dobs (monastic Disciplinarians, sometimes called monastic police forces) do not stem from a place of anger, and while they might appear violent to an outsider, do not fit the metaphysics of (Buddhist) violence.

(Naturally, it's impossible to go over each and every example and counter example. I'm giving the theory of how historic Tibetan society worked.)

With all of that context, I have to say that I have never read from a reputable, non-Chinese source of Tibetan monks murdering local peasants and making serial-killer like objects of their skin. To be honest, it sounds ridiculous even for Maoist propaganda regarding the Tibetan bougeousie. The Beijing government did its best to present an image of cruel and violent monks, often taking the Dob-dobs as their inspiration. In Lhasa there was a whole museum dedicated to the crimes of the Tibetan sangha, with an ugly Dob-dob weilding a massive club to beat a poor woman holding her children. (Get back to me for pictures on that.)

While the Dob-dobs have been known to abuse their position of power (often going so far as to rape young monks through thigh-based outer-course, a horrible abuse of the spirit of Vinaya monastic discipline since it is not explicitly forbidden by the Buddha) it's kind of like any law enforcement system: it's highly dependent on where, when, and whom we're referring to. The Tibetan sangha wasn't very centralized, and the Gelukpa Sangha in the vicinity of Lhasa itself was famously at literal war with itself repeatedly throughout the period of the independent Tibetan government. (And it's worth noting that I only know the Dob-dobs as a Geluk phenomenon. I've never heard of their use in the Kagyu, Sakya, and Nyingma schools.)

/cont'd