The Dragon Age series contains a multitude of decisions (and subsequent consequences) for its players to explore. And while some Dragon Age choices are based purely on player opinion, others consist of actual good and bad choices that the player character can make.
That said, there are a fair number of unpopular choices available in all three games, let alone just one, ranging from "painfully pragmatic" to "just plain evil." Right now, however, it's looking like one of the most "just plain evil" decisions from Dragon Age 2 could very well come back to haunt players who decided to go for a more sinister sort of playthrough.
Dragon Age 2's Most Evil Act
Dragon Age 2's protagonist, Hawke, can be a lot of different things--from earnestly altruistic, to a well-intentioned goofball, to an angry hothead with a propensity for violence.
For the most part, Dragon Age is known for featuring decisions that have no "right" answer, with pros and cons no matter which choice players make. Still, that doesn't mean there aren't a few choices here and there that are quite obviously coded with good, neutral, and evil options, such as how players deal with Orana, or whether they let the Qunari take Isabela with them.
Out of all the choices players can make in Dragon Age 2, one with potentially far-reaching consequences is also arguably one of the most evil: the decision to give Fenris to Danarius when the magister hunts him down in Kirkwall. The quest is "Alone," Fenris' companion quest for Act III of the game; in it, he arranges to meet his sister Varania (who he only recently remembered and learned was alive) at the Hanged Man. Of course, it's a trap. Varania betrays him, hoping that if she leads Danarius to Fenris, the magister will make her his apprentice.
From there, Hawke has two choices: help Fenris fight Danarius and gain his freedom once and for all, or let Danarius take him as a slave back to Tevinter. Obviously, there's no grey area to those choices. Either Hawke does the right thing and helps their friend, or commits a truly reprehensible act and returns Fenris to shackles.
Now, BioWare's Dragon Age canon (which is used in books and comics set in the world of Thedas) assumes that Hawke stood by Fenris during the events of the game. The current run of Blue Wraith comics also stars Fenris, and places him in Tevinter currently, fighting back against the worst of the magister and freeing slaves. That Dragon Age canon has led many to speculate that, being in Tevinter already, Fenris stands a good chance of appearing in Dragon Age 4, which is also thought to be set in Tevinter. However, what happens in a world in which Fenris was returned to Danarius?
The Consequences in Dragon Age 4
If we instead assume that Hawke did the unthinkable and let Fenris be enslaved once more, there's still a good possibility that he'll appear in Dragon Age 4. To start with, he'd still be in Tevinter. And considering the several years between the end of Dragon Age 2 and start of Dragon Age 4 (we don't know how many exactly but it has to be at least 7, since according to the timeline DA2 ends in 9:37 Dragon and Inquisition DLC Trespasser takes place in 9:44), Fenris has plenty of time to break free from Danarius once more and become the Blue Wraith freedom fighter that he is in the comics. That way, there'd be no break in continuity between the games and comics; the only difference would be in how Fenris acts and responds to the player character of DA4.
So that means players would still be dealing with Fenris, but a much angrier and more bitter Fenris. Odds are he'll be especially untrusting and therefore more difficult to work with as well. Essentially, even though players won't be playing as Hawke in Dragon Age 4, their decision to betray Fenris could still take its toll on their relationship with Fenris in future playthroughs. Plus, if Hawke makes another cameo in DA4, assuming they're alive after the events of Inquisition, things could get dicey fast.
Fenris might want revenge. Or perhaps, by the time DA4 starts, he's already gotten it, on Danarius at least. If there's enough time for him to become the Blue Wraith, there's enough time for him to eliminate Danarius, too. Since DA4 is probably set in Tevinter, it seems likely that players will have the chance to reform Tevinter somewhat, and if that is the case, Fenris would be a good candidate to help with that. If the Fenris helping is one who has been betrayed twice over, however, he might take an even darker turn within the game.
Remember Leliana's arc in Dragon Age: Inquisition? Even though players already had the chance to soften or harden her in Origins, Inquisition players also had the chance encourage her towards mercy or ruthlessness. Fenris could definitely go through a similar process in DA4, with players having the option to help him overcome his rage and bitterness, or push him to embrace it completely.
Whatever character arcs BioWare is planning for its DA4 characters, Fenris seems to be one of the most likely characters to turn up in some capacity, along with others like Dorian Pavus and Maevaris Tilani. And, if Fenris is present, it'll be interesting to see the different facets of his character depending on the events of the previous games - though players who abandoned him before might have unknowingly made things more difficult in the future.
Dragon Age 4 is in development now.