That's why she ensnared him: this never will free (30)
There's a little more crazy-jealous talk here. One of the things that sticks out for us is that the speaker never blames the guy. It's the other woman who "ensnared him." It's almost like he never had a say in the whole thing, and just accidentally found himself running off with a new girl. We're not buying it, but we've definitely seen this kind of thing before. This part of the poem reads like a nineteenth-century reality show to us, like if Robert Browning had thought up Cheaters 150 years early.
For only last night, as they whispered, I brought My own eyes to bear on her so, that I thought (33-4)
This is why Browning is awesome. He can make a whole scene come to life in just a few lines. Can't you just see this happening in real life? You're watching the guy (or girl) you like talking to someone across the room, and you stare extra hard, just to let them know they aren't fooling anyone. We're pretty sure anyone who has been to a middle school dance has felt this kind of heartbreak.