I'll be honest, I interpreted Boq looking up at Glinda on the balcony as a sort of resignation.
At this point, he's been stuck for so long. With Nessa, in Munchkinland, and now like this. There's no getting out. He's been through so much only to end up a monster.
I'd also like to drop this quote from Ethan Slater's FAULT interview:
"Even though his heart is shrinking from the spell, it’s really been shrinking the whole time. He’s been losing his capacity to love or see the good in people. So for me, the moment he gets turned into the Tin Man is as much a magic spell as it is the culmination of something emotional, suddenly he doesn’t even recognise Elphaba. He’s been told by someone he knows is lying that she’s a bad person. He knows the Wizard is lying because he knows her, and yet he believes it so strongly that even the glimpses of clarity that she saved his life, that she cared, begin to twist in his mind. He becomes his own conspiracy theorist."
Of course he's mad. He has every reason to be mad. He's scared, and has been broken down so much over time. The transformation was just the final tipping point. Boq has cared so much, and for what? When is it his turn to get upset? When is it his turn to have just a single moment of validation?
I think that, in this scene, it does hit him what he's doing. Probably when he corrects himself from saying Elphaba to saying "the Witch" instead. It was his friend that did this to him, and he's going to kill her. That was decided before he even started his speech.
Looking up at Glinda at this moment, maybe he sees Elphaba. Sees an old friend that isn't there anymore. Or maybe he does see her. The lady he used to love, and they've both changed. He doesn't love her anymore, because he can't. He's loved so much, and now there's nothing left for him to give. Boq has exhausted himself.
Looking up at Glinda, who, in my opinion, recognizes him in that moment, things truly solidify.
He truly no longer loves her, the one who's had his heart for years, who probably kept him going when things got bad with Nessa. He's going to kill Elphaba, who betrayed him in the worst way. By now, he has to know Fiyero is gone. Nessa is gone. His humanity is gone. In this moment, the major thing is that he's resigned to the fact that this is his life now, and that he's going to end that of one of his best friend's.
