so the massacre thing is kind of an interesting add. most of the time i have seen them it says “0 infected killed. Massacre.” just this last mission i got attacked by a massive horde and the orange box popped up “1 infected killed. +40xp. massacre.” According to the wiki page it has to do with killing a certain number of infected within a certain time frame. It is super confusing though when you kill a dozen or more and it tells you that you have killed 0.