The first thing I have to comment on in this episode is Kyle putting on that padding and making Simon attack him as if it's going to help them figure out anything at all. Honestly, what was he trying to accomplish? Was it just a confirmation that the mark really would hurt someone? Because I have no idea what else he expected to get out of it.
I'd forgotten how almost reasonable some of the stuff Heidi says is when she first meets Simon. Of course, that quickly changes as she starts murdering people. Still, understand her feeling betrayed when Simon wants nothing to do with her after Raphael tortured her. I feel bad for her, and I do feel like her story is tragic. It's made very clear that she's never really had anyone who cared about her, and it makes me wish that the show had handled her story differently.
I like that Maia recognizes the similar positions she and Heidi were put in after they were turned. It helped emphasize that Heidi reached the point she did because terrible things happened to her, not because she was naturally a bad person.
Alec and Magnus have an argument about Alec aging that breaks my heart. The fact that it gets a little petty did help lighten the atmosphere a little. Alec getting drunk later in the episode kind of proved Magnus' point that he was acting like a child, but this is Alec's first relationship and he's centuries younger than Magnus, so it makes sense that they'd find themselves in such a situation eventually.
I loved seeing Maryse this episode and the aftermath of her being de-runed. I love that she came to see her kids after and that she included Jace when she was looking for them. We also get the beginnings of Luke/Maryse here as Luke offers help to Maryse if she ever needs it. This episode really helped strengthen my fondness for the show's version of Maryse.
Jace randomly bumping into Alec, who happens to be on a walk despite admitting that he never going on walks, was hilarious to me. Why did Jace not put some effort into hunting for people in places his siblings wouldn't be? Or did Alec's walk also take place in some far flung part of New York that Jace figured he'd never be in?
There's some nice dramatic irony in Heidi thinking that Izzy is Simon's girlfriend. That was a small detail that I really liked.
We see Jem (or Brother Zacariah) for the first time in this episode. Considering they can't use material from the Infernal Devices series, they did a good job teasing those books in Jem's conversation with Clary and Magnus. I do wonder what that scene looks like to someone who hasn't read the books because I imagine that it raises some questions. Jem even says, "That's a story for another day," in reference to him not looking like the other Silent Brothers. In most circumstances, I'd consider that a hint that the show will explore it later, but here, not even taking into account the the cancellation, that's not the case because they can't legally tell that story. I wonder if it felt off to some viewers, but as someone who has read the books, I loved those bits.
When this episode aired, I was still holding out hope that we'd get some kind of glimpse of Tessa too. Unfortunately, we never got that, but I did like the bit of fan service this episode gave us.