Struggling so much, when you just needed to kill two truly evil people. Baka!
Yeah that might be the right answer. Also the nephew, if he didn't care about the ritual, should have helped the couple cover up, especially when it came to the bodies. Help them find bodies, help them delay the offering, fake the bodies, etc. It's better for all of them anyways if everything goes smoothly. If they actually had to kill people, it would potentially make way more issues with the authorities, especially if he wanted the money to keep coming in.