Is it a matter of the degree of the crime like "satsurikusha" is worse than "satsujinsha" and "tairyousatsujinsha" is worse than "satsurikusha"? Or are "satsurikusha" and "tairyousatsujinsha" on the "same" level?
- 殺人 is a relatively objective word for "killing" or "murder".
- 大量殺人 is just "mass murder", as the kanji suggest. The means of killing is not important. It's still a relatively objective term, and Wikipedia articles may prefer this.
- 殺戮 refers to brutal and cruel mass murders, typically using rifles, chainsaws, poison gas, etc. Tabloids and novel authors may prefer a sensational word like this.