Is 'fell dragon' more of a political distinction than a nature one in this setting? Dragons go to war with humans for desecrating their dead and so get called 'fell' dragons. Considering how strongly reacting to something carries over into the dragon form it could also be another way of saying feral.
Not quite. There was one Fell Dragon (who isn't Grima) who humans considered the ruler of the other dragons in the war against the Emperor and humans.
It was considered the biggest, strongest, cruelest and most cunning dragon, but other than that was not considered special. There was no mention of heart forms or manaketeness, so Ryza supposes it's
possible they were all feral, but she finds that very unlikely.
EDIT: Feral has a very specific meaning for manakete: its what happens when the heart and the dragon form come into irreconcilable conflict, driving both to madness. The most common and irreversible way it happens is if a living manakete's dragonstone is destroyed, though there have been cases of Draconic Feedback that got so severe that the two parts of the manakete drove each other literally insane.
What usually happens with a feral manakete is they permanently shift into dragon form and just go on a rampage. According to manakete law, that is the one permitted time for a manakete to kill another manakete: to stop a feral dragon-form, since there is no other way to stop them.
Not to say manakete
didn't kill each other at other times, just that it was against the law.