For a Grey Warden, is love "the death of duty"?

Yeah. I'd never stab Morrigan.

I don't think "Love is the Death of Duty" unless you are playing a chantry loyalist. None of the DA storylines limit you by making actions against your LI a matter of "duty". As above, if you are playing as a loyalist then you wouldn't fall for someone whose life and practices go against your sense of duty. You would be revolted by them.

That being said, all your characters tend to have hold a negative opinion of Blood magic, but still allows for a specialization in a skill that draws on blood to boost your own abilities. DA:I being an exception. For a mage character, there is no blood magic exactly, but there is a "dark" specialization. It depends on your playstyle. Even in DA:I, if you do play as chantry faithful, your character should not find someone who uses abilities you disagree with.

I've never been compelled to play as a chantry faithful in DA:O. So Morrigan's use of "forbidden" magics has never been a problem. Therefore killing her when she is my LI is not a thing. The mentality my male warden has tends to make him more curious about Blood magic rather than repulsed. Morrigan is usually my LI as Liliana is not really appealing and Zeveran just doesn't seem dedicated to the relationship.

Morrigan is my canon Waifu. DA2, Isabella. DA:I, Cassandra. Think I'm just drawn to 'difficult" women. Breaking through the icy shell around their heart is challenging. When successful, they tend to be the most dedicated to the relationship.

Warden Commander Cousland and Amell disagree with this prognosis because Alistair is still alive.