Oh, Magnus reaction to Alec’s marriage. It was delicious. And not for romantic reasons, the witnessing of the loss of the love of one’s life. Not all. It was marvellous for Magnus’ own character.
Look at where Alec and Magnus are right now. They are strangers. They helped each other, have some chemistry, get along well enough, yes. Magnus does like a challenge so Alec being so stiff is actually something that adds to his interest: he was all for seducing the closeted hot guyTM. Yes, he was interested. A lot. But honestly? After one drink night and one “heart-to-heart”, a political marriage would be too much too handle. ESPECIALLY if, like Magnus, you like to keep your distance from anything shadowhuntery, and from messes in general. He was up for romance, not politics.
In fact, we see him backing off the second he hears about it.
I think him walking away and his thoughts on Alec, are at this point to severely separate from Magnus’ heartbroken (and heartbreaking) reaction. What make sense, is that Magnus wasn’t heartbroken over Alec and their potential relationship: was he pissed, and sad? Probably. But he was heartbroken for himself.
For his tendency to always get the complicated one. Because everyone he’s interested in always turns out to be a mess. Because every challenge turns out to be a waterfall of chaos and problems. Because he can’t be in a relationship without it turning into a greek tragedy, without tying himself to the odds of the world. Because he can’t get a guy or girl- a lover, a friend, anyone new in his life, without it being not only a simple love story.
He was standing in front of a romance novel, and in that moment he found himself deep in a political thriller. He was suddenly reminded of Camille and her destructive ambitions, of Will Herondale dragging him into a story way larger than a regular job. Of all the time he wanted a romantic challenge and got to help saving the world instead. He realized not only Clary was the key for something larger than life -that he knew since the moment Jocelyn showed up- but that also Alec carried similar implications.
In that moment, helping rogue shadowhunters committing high treason simply because he let himself knowing people he could care for, AND realizing his boy was first and foremost the next head of the New York institute- he found himself staring at the very reason why he had closed off to love and affection, and retired in everything that was beautiful, yes, but hollow.
There and then, perhaps, he even decided that he had made the right choice.