Some psychologists have published a study that "proves" Internet trolls are sadists. The study is nonsense. As Wikipedia says on trolls: "media attention in recent years has equated trolling with online harassment". That appears to be what happened here -- the study is really about "harassers" not "trolls". That sadism would correlate with Internet harassment is unsurprising.
The study was done through self-identification. Participants were given a questionnaire asking "What do you enjoy doing most on these comment sites?", with the choices being: "debating issues that are important to you", "chatting with other users", "making new friends", "trolling other users", and "other (specify)".
In other words, rather than objective measures based upon participants online activity, it's wholly subjective.
There are two flaws with this approach, one with how people self-identify with "trolling" and the other with how they self-identify with "debate". Those who enjoy harassing would likely describe their activity as "trolling" rather than "other". Conversely, those who objectively do the most trolling are likely to believe their activities qualify as "debate".
The word "troll" was originally coined to refer to those who did so accidentally. Pick any forum with back-and-forth comments, and you'll find lots that is incendiary, off-topic, and emotional -- from people you think what they do is "debate". Few discussions on the Internet qualify as serious debate with reasoned arguments citing unbiased sources. In other words, a discussion on taxes will be filled with emotional appeals such as "the rich don't pay their fair share" rather than citing hard fact, such data from the CBO, on precisely what share of taxes the rich pay. A more rational study would be a blind study that would identify trolls by the statements they make in online forums. And only then would the study try to correlate trolls with personality defects.
Nowadays the word "troll" also refers to people who do so intentionally. Some of these are undoubtedly sadists. The well know troll "Weev" is an example of somebody who seems to be on the harassing side of trolling. But there are also those who do so on the comedic side. For example, the twitter feed of @AnyLevy is full of humorous trolling design to provoke humorous reaction.
I can't predict what a correct study of trollers (both accidental and intentional ones) would find. I just know that the above study is wholly incorrect.
By the way, what trolls me? Bad science.