Why they didn't use the guns in the house.

There were two shotguns in Kitty's closet, I believe. It's my understanding that they felt they couldn't risk their parents noticing their guns were missing, and would rather have them in their own place in case the shit hit the fan.

Mainly, as it was Erik who bought them, I think he felt he needed one in his room to protect him, rather than having to try to fight off Jose and then run into Jose's room to grab one of his.

In an odd, roundabout way, it kinda confirms to me the fear. If they weren't afraid of their parents and just wanted to off them for money, wouldn't they just wait until they were incapacitated and grab the guns out of Kitty's closet?