Matthews was a big and strong guy. Big and strong enough that he could easily overpower a girl and catch them if they tried to escape. I think most women would see he was was too strong to fight/outrun, and just choose to cooperate instead of fight.
Hannah and Morgan, however, may have taken their chances and fought back. So, he killed them. There are a few possible reasons:
1. They injured him and he got enraged
2. He became irate because he felt they were disrespecting him (''You're a woman, how dare you not give me what I want/deserve as a man?'')
3. He just got annoyed because they were being too loud, wouldn't stop talking, wouldn't cooperate
4. He felt that letting them go would be too risky for whatever reason. Maybe they got a really good scratch in and had his DNA on them, maybe they somehow found out his name and/or had other identifying information
5. In Morgan's case: Maybe he's used to passive/cooperative victims and freaked out when he realized she was absolutely not going to allow him to rape her. So, he killed her in a panic. Then, in Hannah's case, he was like ''I've already killed one girl, I might as well do away with this one too''
It's really sick.
I'm a girl, I'm the same age Morgan was when she died, and I'm starting college very soon. This case really scares me and I don't know what I would do if I were in a situation like that. It makes me realize how vulnerable and weak I am at 5'4 and 140-some pounds and not being very fast. I'm seriously considering buying pepper spray now.