That is true,
@Blade . We have to define what exactly a troll is because it's not just confrontation or opposition.
Oxford Dictionary states: a person who makes a deliberately offensive or provocative online post with the aim of upsetting someone or eliciting an angry response from them.
It assumes a lot of motivation on the part of whoever is posting to you but I think the basic examples given in this thread are pretty clear on intent. If you post something in a regular manner, not being angry, then someone posting to you a bad word or calling you a name, there is a safe bet that they're looking merely to agitate.
You can either take the stance that rudeness online is never justified(they're just words; if opinions make you angry enough to curse at people and lose your own dignity, then maybe go do something else) or there is a certain line where someone is acting consistently unreasonable for a particular situation or conversation. Like, maybe one angered, blustery comment is understood as passion, but if the person continues to converse with a ridiculous amount of heightened emotion and insults, then they're no longer given the benefit of the doubt.
I've personally encountered people who post nonsense in a topic to derail the conversation or confuse the other members. I've debated with someone who kept circling a point until it became clear to me that they didn't truly believe or support what they were posting. They were just saying stuff to keep me going. These would count as trolling also but aren't necessarily a "one response and you know" type of encounter.