Even with another woman, there is no motive.
Just look at it from a conventional standpoint.
Two people get together and there is pregnancy. Happens. Mom is religious. There is a kid. They move close, both are in kid's life, both carry on with lives/careers, there is a formal/informal parenting plan, no marriage officially on the way, they live separately. There is no divorce in case of another woman. Both participate in child rearing. No domestic violence.
No reason to kill, in my opinion. Anything can be resolved. The guy is spending more money on a criminal lawyer now than he would have spent on any family lawyer.