Fenris's anger towards Danarius in act 1 is so deceptive. Not that it doesn't exist - it definitely exists, is very real and all-consuming. And Fenris definitely creates a very logically sound argument for why it exists and why Danarius deserves to die and why it would be incredibly insulting to just pay Danarius for his own freedom - ie. the institution of slavery is evil! after everything he's taken from me, why does he also deserve my money?! (Absolutely a fair point. But nevermind that Fenris knows perfectly well that Danarius is already extremely wealthy, and already expending a far greater amount of money having him tracked and hunted and brought back alive than Fenris could ever hope to match.) And I think it all distracts from the fact that Fenris is just not a very ideological person and isn't actually motivated by ideological ideals. Which is what makes him a sensible and reasonable and pragmatic person (unlike Anders who is 100% fuelled by outrage against injustice in the face of every practical impossibility to his plans, and is thus insane (i say this affectionately, please keep your Anders hate/salt off my post)).
There's just a very practical reason that Fenris is so angry in Act 1 and I think it's that his anger is one of a very few things that's keeping him from going back to his abuser. Like, Danarius has gone out of his way to make as sure as possible that Fenris's time as a man free is as miserable and uncomfortable as being his slave, if not more. When you meet Fenris, he's being chased across the filthy backwaters of Southern Thedas by bounty hunters, hounded and paranoid and unsafe at every turn, without access to adequate food or housing or medical care, incredibly lonely and entirely without allies (and who would want to ally with him, when it comes with the strife of becoming a target of those bounty hunters too??). He is living a miserable grimy existence, and he knows that the easiest way to make it stop is to give in. To go back to Danarius - let Danarius be the solution to the problem that Danarius created in the first place, entirely with the intention of bringing Fenris back under his control. And the only thing stopping Fenris from doing that is him reminding himself at every inconvinient moment that he's furious with Danarius and the guy made his life hell and deserves to die miserably. And you think so too, right, Hawke?! Tell him you think so too!
So that anger is important, but the things that Fenris said in it also can't really be taken as a literal understanding of his thought process or his actual desires imho. It's just pretty obvious by the time you reach acts 2 and 3, when Fenris has far more in the way of resources and allies and security, that all his conviction and outrage in act 1 about how he'd go and hunt down Danarius and kill the man himself was an extremely empty bit of hot air. His grand plan for dealing with Danarius in act 3 is 'hope that guy has moved on and forgotten about me so I can meet my sister in peace'. Frankly, he doesn't want to kill Danarius - doesn't want to have to. Much in the same way he didn't want to have to kill Hadriana. He doesn't give a shit about revenge or whether or not they deserve it for their magical crimes. It's just that none of these fuckers will leave him the fuck alone to move on with his life.
