Even though "calmer" is correct, I feel like the sentence would sound less awkward if you used "more calm" instead. Also saying "it" instead of "things" just sounds better to me.
I decided to leave, hoping it would be more calm when I got back.
You also don't even need "-er" or "more" next to calm. Since the situation you were talking about wasn't even calm to start with.
I decided to leave, hoping it would be calm I got back.
Again, it just sounds more proper to me this way.