Napoli were a huge disappointment for their fans once again I'm afraid.

If it weren't for that red card maybe there would have been a chance for Napoli to make a comeback. Mazzocchi is really unbelievable. He entered in the 46th minute and got a direct red card only 4 minutes after that.

Just how can you?
Indeed, Napoli have disappointed all their fans and of course the 3 - 0 defeat to Torino was humiliating defeat because Torino itself is team that is below them.
It just that Torino took advantage of their home match very well, they were able to improve performance and Torino also managed to dominate the overall match after Napoli only played with 10 players due to Mazzocchi exit.
This match also proved that Napoli performance had actually experienced significant decline and it could be said that Rudi Garcia coaching change was also unable to bring about change for Napoli.
The more interesting part is Mazzocchi was signed by Napoli only 3 days ago. This must be the worst start for a player at their new team. Osimhen is out for a while due to national selection so a really bad time period might be waiting for Napoli until he comes back.

You are right that Mazzocchi is new player brought in by Napoli from Salernitana and on his debut with Napoli he immediately gave red card which probably made Mazzarri feel disappointed.
If only Osimhen had not been injured and absent from the match, maybe Napoli would have had an easier time in every match they faced, but the reality is that Osimhen is the main key that Napoli has.
Without Osimhen, Napoli is just like lowly team that doesn't have enough strength to compete this season, just imagine if Osimhen was really sold, what would happen to Napoli.