Hermia accuses Demetrius of murdering Lysander when she wakes up and can’t find him, but he is not actually dead.
Lysander leads Hermia through the forest, but he gets lost. They are both so tired that they just lay down. Puck sees Lysander wearing Athenian clothing and assumes he is the one Oberon ordered him to anoint with the love juice. Helena wanders through, also tired from her chase of Demetrius, and sees Lysander sleeping. She thinks he is dead, and wakes him up. He falls in love with her instantly, and she runs away with him giving lovesick chase. At this point Hermia wakes up, and finds Demetrius and not Lysander. She jumps to conclusions, assuming that Demetrius has murdered Lysander (foreshadowing the duel the two have later over the now distraught Helena.
Demetrius gets annoyed, and says he’d rather “give his carcass” to his hounds. Hermia takes this as confession.
Out, dog! out, cur! Thou drivest me past the bounds
Of maiden's patience. Hast thou slain him, then?
Henceforth be never number'd among men! (Act 3, Scene 2)
Demetrius finally tells her that he did not kill Lysander. She leaves, and he lies down, exhausted from being chased and chasing.
Hermia jumps to strange conclusions about Demetrius. If she really thinks so little of his character, no wonder she does not want to marry him. Her behavior is also symptomatic of her exhaustion and mental instability. She is wandering through the forest at night, wakes up alone, and sees the man she does not want to marry. Her reaction might be extreme, but speaks to her frustration.