Well, in medieval France, the Jacquerie, a peasant insurrection of the 14th century, led to several assassinations, which are related in gruesome (and arguably dubious) detail by chroniclers. The most famous example is that of Jean le Bel, who tells us how rebels killed a knight and roasted him. In the ancient world, many important people died in riots in which the urban poor probably played a role. However, I cannot think of any well-known assassination attempt by a single man (even though it must have happened) before the end of the Middle Ages — the assassination of Henri IV of France by François Ravaillac, a deranged man, could be an example (however, he was not a poor peasant either, his family was actually rather affluent). Just like nowadays, violence in pre-modern times tended to happen mostly between equals (but contrary to nowadays, it happened in every social strata, whereas it is now mostly restricted to the lower class).