Well IOTL he opted against evacuating with the rest of the government to Kiubysheev. He recognized unless he stayed the public would freak out. I know that ON is going to come and say the NKVD straightened everything out, but if the Germans were in a position to overrun the city there would be a mass breakdown of public order and self evacuation east. I'm thinking Stalin stands and dies or only tries to flee when it is clear that Moscow is already lost. Not sure he can make it out of the city in one piece then, but I get the impression his team at the time felt he was the only guy that could hold the country together in spite of his failures of command. At that point I think the country starts unraveling because morale would drop off a cliff once the capital is gone, while Stalin's leadership, as well as those of his generals, wasn't enough to hold the capital and industrial heart of the nation at that point (the Urals as the new heart was only just being assembled). So things could go many ways in terms of Stalin, but personally I think he stays and it is 50-50 whether he dies in Moscow or bails at the last second. We'd kind of need to know the details of how Moscow is overrun to really comment on the situation specifically. If Stalin dies then politically the USSR starts to unravel at this point, because there is no clear successor and there is a lot of bad blood between Beria and everyone else, which Beria knows and will react to, which undermines the ability of the surviving government officials to actually unify. The army isn't going to trust him and may move to remove him. This isn't 1953 after there was more peace in which to work out the details of Stalin's succession, while his team was more unified.