If I understand it right than Lisa is the wife of Trevor, obviously. She wished to have kids (see the painting of the long woman with 2 children) and got pregnant.
Trevor has phsycological issues (see Lisa`s diary) and seems to not want kids, probably because of his issues, afraid of not being able to raise them right. Eventually, he damaged the Baby Name book from Lisa out of anger (see the state of the book).
On a little walk in the nearby forest Trevor killed her (see the thoughts of Trevor in the final scene) with a knife and diged a grave for her.
After that, Trevor threw the knife in the washing maschine and continued to live his life until his inner demons finaly got him (the long woman, which represents Lisa) (the watchfull eyes in the final scene) (Lisa herself).
He had then the option to either run away, unable to ever rest, or end it all here and now with his Second Amendment right.
Decent game.