Adding a 4th factor for outlier Italy:
4. running out of equipment.
Although it is certainly not the only explanation, the overtaxing of intensive care infrastructure is no doubt contributing significantly to Italy's exceptionally higher death rate of 10%. Places with far lower death rates would see their rates spike if suddenly the number of cases requiring ventilators began to greatly outnumber available ventilators. Northern Italy's outlier status is owed in large part to the collapse of its intensive care capacity. This underscores why # 3 in the list below is so critical to what is happening there. There was not just 1 "patient zero" in North Italy, there were 100s perhaps 1000s of patient zeros. This is why Dr. Fauci keeps stressing the utter importance of the United States imposing travel restrictions on China when it did.
Once you add reason # 4, then the importance of 3 and 4 become far more significant together than either of them by themselves. The greater the numbers of cases in a concentrated area, the sooner the intensive care infrastructure will fail, thereby increasing the death rate by many times.
That said, I still believe that all else being equal, # 2 in the list is a big contributor as well. And of course # 1 still goes without saying. Quite honestly, without knowing this for certain, # 1 in the list is probably the single biggest factor for Italy's exceptionally high death rate. The reason is because even if the case numbers were lower, the number of old versus young in Northern Italy would ensure an an equally and exceptionally high death rate.
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North Italy is a perfect storm for maximum coronavirus effect:
1. North Italy has exceptionally high old to young ratio, one of the highest if not the highest in the world.
2. The Po Valley is the most air polluted region in all of Europe; coronavirus is especially hard on air pollution damaged lungs
3. Lombardy has highest number of Chinese citizens of any region in all of Europe. Much travel back and forth between Wuhan and North Italy in November / December / January before Italy imposed travel ban
4. All this exacerbated by the force multiplier of running out of equipment.
