The difference is, I don’t think it would fail in the scenario you put forward, so there would be a consequence. Now if we assume it’s biden, and he’s unlikely to see out another term even if elected, maybe the risk/reward ration swings enough to make it a realistic possibility, but in general I suspect a president assassinating multiple supreme court justices would lead to them being impeached successfully in short order.
I thought the entire concept was jokey, but OK, let me restate my point: Yes, the US government has killed many people, though typically individually and discretely. The president personally shooting multiple supreme court justices as Sickos suggested would be seen to be several levels above that and probably be too extreme for even democrat supporters to stomach. The dems also seem to like to be seen to be playing fair, which suggests they wouldn’t even contemplate that approach.