Obviously the workload they were putting on you was way beyond a rational amount for one person. If you're willing to talk about it, can I ask if you considered simply refusing to do more than "one person's" amount of work and asked them to prioritize all your tasks so you could work on the things they deemed most important? Obviously this could lead to you being fired, but if you were already set to quit, the outcome would have been the same (except perhaps you'd be eligible for unemployment insurance).