I do not agree with those on here who think Thomas killed Kerstin deliberately. I don't believe the evidence points to that. By the way, those posting that he did need to be aware that, although it may be unlikely, he could sue them for libel under UK law and, unless they could prove that he deliberately killed her, they would lose.
The judge was essentially of the view that Thomas overestimated his own ability and underestimated the conditions. He had no formal training and had never taken an alpine course. He knew how to move fast, but didn't know how to lead and lacked the skills to know what to do when things went wrong. And he did not understand his girlfriend's level of ability. The judge was of the view that his behaviour was not that of an experienced alpinist with training in rescue methods. The judge's answer to the question some have asked as to why he didn't wrap his girlfriend in a blanket is that he did not leave her on flat ground as he claimed, but that she became so exhausted that she was not able to climb through that section and that he tried for 1.5 hours to get her up but he lacked the skills and knowledge to do so.
If the judge is correct, the question is at what point do overestimating your own ability and that of your partner, and underestimating the conditions tip an incident over from an accident to a criminal offence. The judge basically held Thomas to the same standards as would apply to a professional guide. It is now for the appeal courts to decide whether that was appropriate.
By the way, we don't just have the rescue team saying they were both climbing when the helicopter observed them. According to the judge, there was video footage which showed that they were both still climbing, although he (the judge) says Kerstin was clearly struggling. He says that in 5 minutes she only manages to move 20cm upwards in quite manageable terrain.