Who gets to decide what someone was really wronged? Me? You? "law"? Your own parents hating you cause a religion said so is more than enough for being wronged. I don't even need to mention the events of circle.
Societal consensus, really. You, me, the "law", parents, religion, counter-religious sentiment, the whole shebang. That's what culture and ethics are in the first place: general consensus.
There is no ultimate arbitrater unless you believe in such a divine being. There are no objective standards outside of our cultural creations and understandings. Otherwise, it's just people- not just the victim, not just the prosecutor, not just the accussed, not just the judge.
That said, Anders was not even set on vengeance first. Events such as what Karl went through caused him to twist his original intent. Have you actually made him a 100% friend, romanced and did his act 3 quest? He actually mentions he will have retribution even if it means he has to die for it. Also prior to that he considers the Chantry a greater enemy than the Templars, why I don't know. Maybe it has to do with the fact that Chantry is making Templars "holy" and without it the people can see they there is nothing Divine about Templars.
I've seen Anders from different relationship paths. I've never gotten the impression he considered the Chantry a greater enemy than the Templars: before Act 3 he's pretty much anti-Templar rather than anti-Chantry, and during Act 3 he's anti-anyone-who-doesn't-agree-with-him, which is just about everyone including many mages. He blew up the Chantry to provoke the Templars into a massacre, not because the Chantry itself was his target: his intended recipeint of the terrorism was Meredith, not the Divine or Chantry institution.
His willingness to die is not in question. Whether his view of retribution is justified is- something he himself struggles with between the various relationship paths.