I think I've also always thought that the children were the first victims because it was easier if they were asleep, than if they were awake and looking at you, maybe even talking to you.
That is perhaps giving the killer more sensitivities and feelings than they actually have/had.
I believe Bamber to be without conscience Hartley whether they were awake, looking in his eyes, made no difference to him.
If the sight of a small child sucking it's thumb in their sleep doesn't put you off, nothing will.
The judge described him well "evil beyond belief"
You obviously emote Hartley - Bamber doesn't!
It's hard to explain unless you've experienced pathology first hand.