...whatever we think they mean. The connotation of any particular word is simply the imagery and symbolism that is evoked in the subject that is listening based on their experience and education.

That said, there are technical terms that are applied to specific fields of study, such as mathematics and science, that have a higher level of specificity when invoked in a specific context. But the word being discussed does not fall under this category.

In the case of using the word massacre, the speaker hopes to evoke images of wanton and unjustified murder and atrocities. As such, it is an emotionally charged term that has a polemic intention. So I guess you could say that massacre is a subjective term, in that it requires judgement between when a murder is malicious and when it is justified. The intent of the perpetrator must be judged to determine whether the word is actually applicable.