The father seemingly did not make efforts to step up and be in his kids lives even while their mother was in jail. If he was less of a pathetic excuse of a man there would be a much greater chance that child would be alive.
This does not seem like a case of father getting screwed over by the system(which is a massive issue) more like a father who did not even try despite having knowledge of the character of the mother. If he had tried to stay in their lives while she went off the rails he would have got custody of those kids(unless there were very serious issues about him unknown to us).
No this does not make him anywhere near the level of filth that the mother is. He is however another adult in a long line that failed these kids and the fact they are his own means if he had even the slightest bit of integrity he should feel really ******* s**t about himself.\
We know nothing about this man or the situation that has led to him being estranged from his children. I think we can assume that he is no saint, and he could have done more to protect his children, though asking "why didn't he stay?" is straying close to "why didn't she leave?", in that it probably has a complex and ultimately pointless answer. Complex in the sense that there are probably multiple reasons. Pointless in the sense that it serves as a distraction. He is not on trial here.
He's a father that has lost a child. He does not deserve the vitriol that has been sprayed in his direction, including on Facebook groups ostensibly against domestic and family violence.