its canon, its mentioned in a comic too. Also the articles about Mage Templar war that mention Anders also mention how he became possessed and killed templars and then moved to Kirkwall. IT IS CANON. deal with it.
Gaider said years ago the comic in question wasn't canon.
He even mentions it in this brief interview on this very subject.
http://www.reddit.co...tus_of_ogb_and/
Which can be found in the blog question but assuming you don't want to read?
Here http://dgaider.tumbl...uch-pull-a-lead
So i don't care what the comics has say, what an article said or anything to that effect.
The daddy of DA lore said it never happened.
It never happened.