✧ :Mental or emotional strength
This is somewhat obvious, but Dark Donatello is indeed rather intelligent, and very quick-witted, somewhat more so than his brothers. He has a knack for recognizing deception and trickery, then either exploiting it to his advantage, or turning it against his enemy. Emotionally, though, his only strength is how well and how long he is able to keep up the facade of inner calm before the bottled up rage and turmoil breaks the surface.
✯ :Way they could see themselves dying
One of several ways, and the one Don sees as the most likely is going down fighting Darius. That is awfully general though. A more specific situation would be running from his ex-boss while hearing the terrified screams of his brothers as they perish one by one, then being slowly hunted down, constantly toyed with, right up until the killing blow.
★ :A socially unacceptable thought they’ve had
There’s too many of these to count (Several of them may be considered socially unacceptable in 2015/early-mid 2000s, but considering Dark Donatello is more often than not in the timeline of 2105-2107, I will be making up a few of these ‘socially unacceptable’ ideas). One of many is insulting every creature in New York (anonymously of course) and causing a monstrous, city-wide riot so that every being could be enveloped in his own kind of chaos. Many of his thoughts are rather incestual, and while this has passed in 2105 as somewhat socially acceptable, it is still considered unappetizing.
Another is bringing down all of his enemies in public, which are more or less the turtles and Dunn, and broadcasting it on every screen he could, cursing and shouting and threatening, ranting about how nearly everyone had the high life while he was forced to suffer, making promises to tear down the city worse than the Triceraton Massacre of ‘04.