I'm learning abt punishment and crime that happened in England and how persons had to undergo ordeals by trials. Question I'm having trouble with is why people and whole village in general had to come as first finders and be in mercy when they didn't observe finding body?

And what were ideologies of this society that contributed to need of such a duty ?

http://www.duhaime.org/LawMuseum/lawarticle-120/crime-punishment-in-medieval-england.aspx

that doesnt answer my question, it don't say anything about first finders.

What does "first finders" mean?

There were some truly horrendous punishments in those days. Did you ever watch the movie Braveheart?
http://www.google.com/search?q=draw+and+quarter&oq=draw+and+quarter&aqs=chrome..69i57.3136j0j7&sourceid=chrome&es_sm=122&ie=UTF-8

In medieval England, when a crime was committed and the victim was found dead, the idea of a communal responsibility emerged. According to the legal system of the time, anyone who found a dead body was required to report it to the authorities, and they were often held responsible for determining who might have committed the crime. This concept stemmed from a combination of communal responsibility, religious beliefs, and the need to maintain order within the society.

The primary reason for requiring the whole village, or at least a group of people known as the "finders," to be involved in the investigation was to ensure that crimes were not overlooked or ignored. It was believed that involving the community in the process would increase the chances of identifying the perpetrators. Additionally, this approach put pressure on the villagers to cooperate and assist in resolving the crime.

The society of medieval England was influenced by several key ideological factors. Firstly, religious beliefs played a significant role in shaping the views on justice and communal responsibility. The dominant Christian values of the time emphasized the importance of obeying God's laws and looking out for one another's well-being. This notion was reinforced by the idea that failing to report a crime was a sin.

Secondly, the feudal system that was prevalent in medieval England created a strong sense of collective responsibility within communities. Everyone had assigned roles and obligations, and this extended to maintaining law and order. As a result, the society expected each member to contribute to the functioning and welfare of the community, including participating in the investigation of crimes.

Lastly, the lack of a centralized police force and the limited resources of the judicial system meant that communities were often left to handle smaller crimes on their own. This led to the development of a system where the community was actively involved in maintaining order and resolving offenses.

In summary, the requirement for people and whole villages to come forward and report a crime they did not witness was based on societal beliefs that emphasized communal responsibility, religious obligations, and the practical need to maintain order in the absence of a formal police force.