she was probably threatening to leave him because of abuse or drug use, etc....he couldn't handle it...so he killed the kid, then killed her, then killed himself.
seen this scenarion 100 times where the wife is going to leave the husband and he can't handle the fact that his kids are being taken away or the family is being broken up, so rather than just deal with it like a normal human being, he figures if HE can't have the kids, she won't have the kids either...and he kills them.
look at the guy from the UK who was vacationing with his family in Greece...had found out his wife was having an affair and so he decided to jump out the balcony window of their hotel with his two young kids under his arms...he lived, his little daughter lived, but his son was killed upon impact....
pathetic and sad