To kill your wife, your own young children you have to be insane and overcome with something seriously wrong.
So without getting to know the guy, it is best left well alone. Such individuals have a definite detachment from
reality, possibly with psychopathic tendencies. So any "conversion" etc. would need to be regarded with scepticism
as a possible way to excuse ones behaviour, rather than coming to see the tragedy of what has taken place.
Though this might sound brutal, the death penalty seems appropriate, where the individual can make their peace
with God, but they are removed from society, seems true justice.
My take on forgiveness, is how could anyone trust such an individual when they have betrayed those who truly
love them and look for total security and protection from them, which they have completely betrayed.
And in truth this behaviour is very rare because it is so against our instincts and biology, as funnily enough
any disposition to follow this behaviour tends to die out, because all the children never grow up to produce their
own kids.
So I personally could forgive such an insane person, but I would not trust them, ever again.