I read those articles too and got the same impression as you. IIRC, RB claimed he offered to take DM in (I inferred it was likely at MB's request), thinking to provide him with some stability and a normal happy family life. He probably did not have any particular hostility to DM at the time -- remember how DM's flight instructor (or one of them) described DM as a bright, sweet-natured, personable kid? When he chose he could present himself well.
RB took him in from around ages 14-16 or 15-17, I don't recall exactly but that's close. We know that he was struggling in school at that time (dropped out multiple times), it appeared he didn't have real friends, but he did become close to some of his cousins - one in particular, a girl much younger than himself (notice a pattern here?), and we don't know what other problems DM may have been exhibiting that led MB to want someone to intervene. Perhaps she/they had sought professional help, we have no information whatever. But most parents with the means do seek help; whether they seek effective help, or do it wisely, is a separate issue. It's very unlikely that DM did not display worrisome signs early on, if not in childhood, then at puberty and beyond.
My speculation is only that, of course. But if DM had some behavioural issues (I'm the one who theorized about the possibility of FAS, but that can't be proven one way or another, though the parallels are striking) these might not have become evident immediately, but rather gradually, over time. RB had 5 children; he would have recognized abnormal behaviour and symptoms, maybe more readily than DM's own parents, who had no other child to compare him to. We know from DM's letters that this cousin (RB's daughter) was important to him, and RB may have grown increasingly concerned over DM's influence on her. The antipathy that was evident in court preceded DM's known criminal career by a considerable length of time, IMO, and probably was rooted in RB's own observations and interactions over the period where DM lived with his family. It's harder to keep up a mask when you live with people than when you charm intermittently. The mask slips from time to time. I think of the famous Oscar Wilde novel, the Portrait of Dorian Gray. A scary but prescient book.
