Not just common Tibetans, but many monks were essentially soldiers. Many Tibetan monasteries were fortresses with armies of their own. They used to fight other orders and monasteries. It's no coincidence that Potala palace looks like a fortress.
Tibetan never had any problem with slaughtering enemies. The hesitation was more to do with whether they could afford to openly fight commie regime or not. As it turned out they were to backward militarily to take on commies.