I don’t care what you say people, that fight was a blessing. It was a real, with facts and a well created fight.
Yes, Magnus dragged his ass but because he is furious and he is going to lash out, of course. Also, I understand Alec position: after everything Magnus been throught, he didn’t want to put more weight on his shoulders. As Alec’s basic instinct, he wanted to protect Magnus.
And of course, Magnus was going find out. It was just a matter of time. He, as a High Warlock, is going to protect his people and the fact that the sword is missing, that is a clear threat. The fact that his boyfriend couldn’t tell him so that he could be prepared and couldn’t be honest with him, he has more than one reason to be mad.
This is a political fight, people. Interest colliding, as well as two different worlds and completely two different people. What did you expect? That this was going to be smooth sailing for them? No. It was long time coming.
This was bound to happen. Something like this will make a breach between all of them, especially Magnus and Alec. Because they will have to make some decisions: protect their own people while trying to save their relationship or simply be done with it.