You may very well be right.
Murder suicide is probably more likely to involve the mans own children than another mans children.
I think that the mental disturbance is deeper and involves depression more than in the case of murder of children of a previous spouse where in the murder of the previous spouses children it is likely to be purely a rage type of murder.
Purely on my experience in reading of this kind of murder suicide where in the children are murdered it usually follows the loss of the mans job and he is despairing over not being able to support his family or the wife is divorcing him and he can not accept the loss of his children.
Any further news of this case appears I would appreciate a ping to the thread.
That is also my observation. The cases of a man killing his girlfriend's other children seem to be situations where just finds the child/ren a nuisance, especially when the idiot mothers decide the new sex partner should be a free babysitter.