unless the man who killed his children had a mental disorder, IMHO, he was a weak coward.
A certain percentage of depressed persons have been long mired in drag-ass depression that they haven't the gusto to kill themselves. Then those persons get an anti-depressant drug that merely raises their energy level and that is enough to "elevate" them to act on suicide. There is no reason suicide embraces murder. That's got to be some connection to pointed hate & revenge, coupled with the totally self-centered evil darkness of knowing you will
feel no blame,
feel no remorse once you've offed yourself
... Once the mind goes to that place, expanded danger abounds. Additional evidence of evil is the fact that he hid his murderous desperateness so he could get his trusting young sons all to himself. Wrong, wrong.! Most certainly his wife & father-in-law have been punished.