Ideally, poor little Sheila should have been taken away from June when she clearly couldn't cope with being an adoptive parent; and the Bambers should not have been allowed to adopt another child.
I feel bad for June, because she must have been suffering, but the well being of children always must come first.
Jeremy's parents married about a year after they had him. In a perfect world, he would have been in foster care until his biological parents´situation was stable and they could claim him back where he belonged.
Growing up, did Jeremy know that his Bio parents had married and had children - his siblings? If he did, he must have felt ousted and discarded - and it might have build up a rage in him.
I am not trying to excuse anything (IF he did it), because it cannot be excused, just speculating what might have been.
Strangely, I know a man who has a very similar history as Jeremy. He was born in Denmark, but his mother was English. She was a young girl, and was sent to Denmark by her upper class parents to give birth and to give away the child for adoption. She later married the not so upper class father of her child, they had three more children.
When this man I know was 27, he saw an ad in a newspaper, an English couple were searching for their son, born this and this date and year - it was him. He does have contact with his biological parents now, visits them frequently, and is not angry, but I do think it gnaws at him being the odd man out.