Final answer:
Oberon aims to punish Titania and gain control of a changeling by making her love a vile thing; Hermia upholds chastity with Lysander; Lysander falls for Helena by Puck's error, leaving Hermia feeling betrayed as her dream metaphorically hinted.
Step-by-step explanation:
Oberon wants Titania to wake and fall in love with some vile thing because he desires to punish her and obtain the changeling boy she guards. This plan involves using a magical flower to make her fall in love with the first thing she sees upon awakening. Hermia insists Lysander sleep a little ways from her because she wishes to maintain propriety and chastity before they are married. Puck anoints Lysander's eyes by mistake, thinking he is the Athenian Oberon mentioned, causing him to fall in love with Helena. Helena reacts with disbelief and suspicion to Lysander's sudden love for her when he awakens, thinking it a cruel trick being played on her. Hermia's dream, where a serpent eats her heart while Lysander watches, reflects reality as she experiences a painful betrayal by Lysander who withdraws his love for her.