It is the deliberate killing of a large group of people, especially those of a particular ethnic group or nation.
Answer:
A locked door can symbolize opportunities that you have not worked towards yet, or opportunities that you haven't been able to achieve. Or, a goal you haven't been able to achieve.
Explanation:
A locked door. Locked doors are doors that need a key to be unlocked, right? So, if the opportunities/goals that you have are behind that door, you need the "key", which is the hard work and effort you put into achieving that goal.
Given that 'blunt' means 'not smart' and that 'mettle' means 'wit; intelligence', I believe that the statement that best paraphrases "What a blunt fellow is this grown to be! He was quick mettle when he went to school" is "This man, who was smart in school, has grown up to be unintelligent".
