A little about love, a little about honor and parabatai.

Well, unfortunately, despite their angel blood, Shadowhunters are just people. They have free will, and as such, are prone to flaws and mistakes. So yes, this could destroy them — if they were above such things, Tessa wouldn’t have had to take the steps she did (clamping down on her feelings for Will, etc.)

Tessa believes that Will needs Jem, and Jem needs Will, and that she “cannot sever them, one from the other — she cannot do such a thing.” Will says that he and Jem are one person; their souls are knit together (he’s quoting the Bible, the story of David and Jonathan,*: “And it came to pass, that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul. Then Jonathan and David made a covenant, because he loved him as his own soul.”) Jem says basically that he has devoted his life to Will, to being the one person “in whose eyes he can find grace” and that Will therefore has to be worth it.

That’s a lot to come between; and Tessa doesn’t want to; she’s actively working to put their relationship ahead of either of their relationships with her. But if Jem were to find out about Will’s love for Tessa in some other way? If Will were to realize that it’s not that Tessa “can never love him” but that she won’t let herself? Well, that’s why there’s another book and it doesn’t just all end here. That, of course, and the GIANT ROBOTS.

*which basically forms the basis of the parabatai relationship, though the words of the actual vow are taken from the story of Naomi and Ruth. If aught but death part thee and me, etc.