I understand that in practical terms a guaranteed conviction from a plea deal is a faster and safer bet for the prosecution but this grub is still getting off far too lightly for the murder of two women.

What exactly makes this manslaughter and not murder?

"We started to argue. I controlled her with my hands but what happened next is completely forgotten. But I did bring her under control with force,"

He might have not planned to kill her initially, but in the end it definitely seemed to be his intend given the circumstances of her death and the fact that he stole her money afterwards to fund his low-life gambling addiction.