Final answer:
The word that best fits into the sentence is 'invective,' as it conveys the idea of abusive language that could lead to character assassination, which should be guarded against.
Step-by-step explanation:
The word we are looking for to fill in the blank for the sentence, “Someone who feels that passionately about the issue must guard against just criticism of the opposition becoming mere _____ or character assassination,” is ‘invective.’
Invective refers to abusive, reproachful, or venomous language used to express blame or censure, which is exactly what should be avoided according to the context provided. The other options do not match the meaning required by the context of the sentence, which suggests that criticism should not devolve into senseless name-calling or attacking someone's character without basis.