...then wouldn't they be of different genera, rather than different species?

I do not wish to be demeaning in any way, but I believe your logic behind grouping the two species in the Trollus genus would be akin to grouping humans and chimpanzees under the genus Homo. Trolls are active and systematic in their dismantling of their prey, while those who merely produce profane expectoration would be more properly called "Flamers".
Of course, it may also be possible that we have identified the two species as being anatomically similar for so long that they were classed in the same genus, in a similar manner to Brontosaurus being equated with Apatosaurus until recently.

So in much the same way as Brontosaurus being given its own genus once again, I propose that Trollus vulgaris shall be reclassified as Flammasermonis vulgaris. The genus name means "flame speech" in Latin, referring to its incessant habit of regurgitating inflammatory snippets of dialogue. From personal observation, Flammasermonis also has less developed and dextrous forelimbs, a smaller braincase, and a tendency to attack even innocent parties without provocation. Trollus malevelous, of course, shall remain within the Trollus genus.

... the vague meaning of the cladistics comment. Though that does require 'troll' to either be non-monophyletic (a group that aren't all more closely related to each other than anything else), or a very high-level term (like 'mammal' rather than 'dog'). Which...

... actually, either of those would make perfect sense. This is the Internet, how many breeds of troll are there?

hS