What appears to run in the family is recklessness. A reporter who covered Pistorius in the past recalled his driving at 140 mph. He also was involved in a serious crash in a boat, after which he supposedly vowed to tone down his wild life. I don’t know any of the details of his brother’s accident, but I wouldn't be at all surprised to learn that he was driving over the speed limit, under the influence, or in some other manner that contributed to the death.
On a cycling forum, I think most recognize that elite athletes (though I hesitate to describe OP that way) very frequently live on the edge, taking major risks both out of as well as in competition. This "I'm invulnerable" attitude, surely, makes it more likely they will not only have a gun, but threaten someone with it, and probably also makes it more likely that they will pull the trigger in a moment of rage.
I do find it interesting that the detective's murder charge came out before the bail hearing, while his brother's charge comes out later. Both involved incidents fairly far in the past.