So Blackwall has a past. Many people do.
Yeah, but "a past" could mean just about anything. In this case, as Hanako said, he killed an entire family for gold, then let his soldiers take the fall and ran. I mean, it's up to each person whether they think he can be redeemed or even if he should try, but I don't know why you find it surprising that given that (and other, more subjective reasons) a lot of people wouldn't be interested in his romance arc.
Personally, I always keep him around, since at best, he really wants to make up for his mistakes and at worst, he's of more use alive than dead. But I find the idea of romancing him... rather disgusting. Even more so because of how he treats the Inquisitor in the romance. Hopefully I don't derail the thread with this.