Kind of a grim question to answer, but I'll tell it like it is. A massacre is what just happened a week ago in Aurora, Colorado USA. A man with several guns entered a movie theater playing the newest Batman movie and killed 12 people and wounded over 50. This happened only a few miles from where I live. Many innocent people were shot by a madman. That's a massacre. They couldn't fight back.
Carnage is similar and means the slaugher of a great number of people. In Word War One, when an army charged another and was gunned down by machine guns, this was complete carnage.