The thing is, he has been convicted in a criminal court for murder. His defence wasn't enough to diminish that, and the conviction carries a sentence of far more than six years.
If one accepts utter remorse and becoming 'a broken man', one still can't reduce the sentence, as the conviction still stands. Mitigation can only do so much, and he didn't plead guilty from the start.

The judge (the same one as before) seems to have enormous sympathy with him, and appeared to want to spare him as much as possible. She kept emphasising that he is a broken man and his athletic career is finished, he has 'suffered' etc etc. Is she forgetting that he shot several times and murdered a young woman, and that her parents must be far more than 'broken', and 'suffering'?.

I have worked with prisoners in UK and many of them had received six years (or more) for burglary or drug-dealing. This would only put them in a Category B prison, not a Cat A for murderers and long-term inmates.