I'm pretty sure it was more than just one 'edgy talk' with his GF, but one of many concrete examples of his attitude towards taking risks and especially taking risks that affect others who did not sign up for the risk.
I saw an article that implied that SBF confirmed on the stand his willingness to do the coin-toss, but I couldn't find it to confirm.
Perhaps the biggest evidence is that he went to trial instead of taking a plea deal. At the very least, he put his parents and family through an awful and unnecessary ordeal, just because he thought there was a chance he could convince a jury. But, in reality, it was not even close.