How to Unlock Mass Murder Achievement
Strictly speaking, this achievement could be done in any chapter with Birthlings (the exploding little monsters).
Just get three of them close enough together so that they are in range to be killed and explode after one gets downed. Killing one will set off a chain reaction and unlock the achievement.
However, in “A Morbid Dance in the Second Line” (scenario 8) you meet a confused man towards the end of the level who will start a fight and will spawn three of the little ones in perfect position. Just shoot one and watch them all explode.