Because those people in Chicago aren't part of a group being fired at indiscriminately, which is what the noun massacre means. Have you bothered to look up the definition?

mas·sa·cre/ˈmasikər/
Noun:   
An indiscriminate and brutal slaughter of people: "the attack was described as a cold-blooded massacre".

Nowhere in there does it say what they have to be doing or whether they caused the situation at all.

Now the verb definition does say a large number of people, but who is to decide what that number actually is. If it is students on a college campus I'd say 4 is a large number.

Really you just sound like a butthurt person at this point, but maybe your definition of butthurt requires more than 4 un-incitefull comments.