"Abuse" isn't ambiguous at all. Anyone with a lick of common sense can easily distinguish criticism and abuse.
Criticism is when you civilly express an opinion of someone or something else, or an action or statement they've made. Criticism is calling someone out and saying you expect more, or have X problem with Y statement, etc.
Abuse is when it moves into personal attacks, or calls for others to engage in personal or physical attacks. Criticism is fine if you disagree, but threatening physical harm or violence to someone isn't criticism - it's a threat. Digging up someone's home address and posting it online isn't criticism. Calling the police to raid someone's house with a SWAT team isn't criticism. Asking if someone would like to be raped because it's a "funny joke" or "satire" isn't criticism - it's threatening and abusive behavior. Any school psychologist can tell you which kids are joking around playfully and which kids are engaging in bullying, and those terms don't suddenly disappear just because we added 20 years to everyone's age.
A movement like GamerGate has many many people in it who willfully engage in what is clearly abuse - and yet the other members who claim not to support this kind of harassment or abuse have done absolutely nothing to stop it. If you're upset that your movement is branded as a bunch of harassers, maybe you should spend more time with the "good" people publicly calling out the ones doing the harassment in your name and asserting some kind of authority.
There's no ambiguity at all - you just want to pretend there is to avoid any responsibility.