The manga I'm looking for is about a world where humans can "coexist" machines. A person can make a pact with a machine and have his or her wishes granted. However, in exchange that person can never fall in love. If the person falls in love, the machine will turn against its contractor and kill the object of that person's love and eventually that person himself. There is a group of people who are able to control their machines and kill these rouge machines. The protagonist is the leader/head of that group.

I've been looking for the manga for awhile and would really appreciate it if someone can help me find this manga. Thanks!

Sounds like Mebius Gear

Yes! Thank you !!!