I think Vader was feeling a deep and supreme disappointment at seeing Obi-Wan again. He’s been thinking of almost nothing else for ten years, wanting revenge on master Kenobi, the warrior that bested him in battle and maimed him completely, and he was probably imagining some intense rematch in which he would overtake Obi-Wan as he was in his prime. And instead he’s met with this old man who hasn’t held a light saber in ten years, running away from him, no desire to fight him, none of the old Obi-Wan spirit familiar to him.
It makes total sense to me that Vader let Obi-Wan go. To kill him then, as a weak man who had no interest in fighting him (and barely could) would not have satisfied his desire for revenge. And that disappointment is probably going to FUEL his ANGER and HATE and omg so excited to see these next few episodes
