Brain Games... Detective or criminal....

A B C D are four friends. Here A&B and C&D are couples. But here is one twist..... A loves D and B loves C whereas Both A and B are cheating in each other and planning to do murder....and here both C and D are helping them... But here A B and D don't know anything about each other but C knows everything... So here's my question to all how will you solve this... Without killing or murdering plans...?