The most basic answer is that this picture of the Middle Ages which is still propagated is incredibly inaccurate. Whilst it is not accurate to say that the medieval period was very peaceful the violence, superstition and chaos is a stereotype containing much bad history. Let me give three stereotypes of the medieval period which contributes to its bad reputation.

1. 'The Church' was highly oppressive and tried to control individual beliefs making great efforts to find out and harshly punish deviations from dogma amongst the ordinary people.

2. This period was one of superstitious witch burnings and mass hysteria which meant that many people, women especially, were constantly in danger of being accused and executed on flimsy evidence.

3. Torture and especially harsh punishments were endemic, far worse than other periods of history, and could be carried out on a whim.

All of these are commonly held beliefs and are either wholly or largely wrong. The idea that the church played an active role in the beliefs of every peasant or burgess, or even wanted to, is dead wrong. It was after the reformation/counter-reformation in the Early Modern period that such minute control became important (if you want a really interesting account of beliefs dug up during this time of increased scrutiny read Ginzburg's 'The Cheese and the Worms'). Things like mandatory church attendance laws are a post medieval occurrence. That is not to say that heretics were not pursued but that this was not done particularly systematically and the goal was to convert people back rather than just kill or torture them. The wars of religion post reformation were also incredibly bloody and notable for burnings of people considered heretics in large numbers.

The witchcraft trope is even more egregious as the most common Medieval ecclesiastical approach to witchcraft was to deny its efficacy. A common trope in exempla from the High Middle Ages is a priest demonstrating to a woman who claims to fly at night and can travel through solid objects that she cannot by pushing and beating her against a locked door. The most influential book on witchcraft the Malleus Malifecarum was written in 1487 (which is at best the very late Middle Ages) and grew in popularity after that in the Renaissance/ Early Modern period. books such as Strix warned of the reality of witchcraft using all the classical erudition of Renaissance Italy. The large scale burnings and hysteria were similarly post medieval but are grouped into this period.

Torture and harsh punishments are not unique to the middle ages. Torture was an integral part of the Roman judicial system (at first for slaves and non citizens and extended up the ranks during the late empire) and Rome also had their share of very harsh punishments (crucifixion, obviously, but their punishment for parricide is also really great in a morbid way). Similarly the post medieval period saw a propagation of spectacular punishments used to keep order (read the opening of 'Discipline and Punish' by Foucault to get a flavour). The so-called 'Bloody Code' in Early Modern - Modern England also shows some of the harshness that could be found in the post-medieval period. Compared to this some medieval legal codes, especially in the early medieval period, are oases of leniency: even murder was often resolved by accomodation and the payment of an honour price. (examples can be found in the Anglo-Saxon Laws of Ethelred or compilations of Irish Brehon law. Moreover the commonly cited trials by ordeal were not used as torture but as a form of proof which someone could submit to. Medieval laws are hard to categorise as a whole as they were very diverse (especially secular law) and intersected by customs specific to different areas. Nevertheless their harshness or arbitrariness was not greater than periods before (although there is great variation and you might say some of the executions of animals were very arbitrary).

In more general terms the Medieval period does not really stand out for violence. Indeed the criticisms made of typical Latin medieval Europe are quite close to those made in that period of those peoples that they were colonising: that they were violent, superstitious, lawless etc.

The wrongness of the portrayal of the Middle Ages is not an accident but rather a trend of labelling stuff we find bad 'medieval' which dates from the Renaissance. This trend was fed by thinkers from the Renaissance to modern period who opposed their learned modernity to a superstitious past period. Medieval was also used as a way of criticising practices which a person did not like even if they emerged much after the middle ages (especially amongst enlightenment figures). Historians have shown these narratives to be at best grossly oversimplified. This can be seen in the identification of a series of pre-Renaissance renaissances (Carolingian, 12th century etc.) which highlighted things thought particular to the later renaissance were around much earlier. I would recommend 'Decolonising the Middle Ages' (which is a whole issue of a journal dealing with ingrained misconceptions about the period and how they might have come about) for more reading on this subject. Unfortunately the stereotyped view of the medieval period has persisted in popular culture and is even taught in schools despite being inaccurate.