i just realized something that makes this scene infinitely worse than it already was–alec’s parabatai bond with jace.
alec wasn’t just visually witnessing the man he’s in love with kissing someone else, there was some part of him that probably felt jace’s reaction to clary as well.
he could likely feel jace’s panic–a frantic heartbeat–when they were heading back to the institute, then jace’s absolute relief when he saw clary again. he could see jace ignoring him in favor of clary, could hear jace telling her that her safety was the most important thing…. and if all of that wasn’t enough for him to deal with, then the kiss.
this is alec feeling his heart break, because he also feels his parabatai’s happiness.