"Why are Tier 1 and Tier 2 members of the Demon Race left alive? Don't you normally kill them? Are they kept around just for survival training?" a student asked.
"Aside from survival training, there are two reasons. The first is to confuse the High Tier Demon Race; low-tier members of the Demon Race lingering nearby pose no threat to us. Killing them all would look abnormal and draw the attention of the High Tier Demon Race, causing more harm than good.
The second reason is that killing them is a waste. Not every member of the Demon Race yields Magic Energy Points when killed. You only gain something by killing members of the Demon Race whose tier is at least one lower than your own.
Chapter 53