Yes, I'll agree it was telegraphed but it goes against who she is. I won't argue that she wouldn't become a tyrant — she clearly showed how she was a tyrant in Meereen. The difference is, she was always about protecting those that were "defenseless" and slaves.

In King's Landing she was just burning indiscriminately. She was hitting the downtrodden, the soldiers, even her own soldiers (including the former slaves), with no concern that she was destroying "her" capital city. In fact, she basically sent a message that she should not be queen and set up what is likely going to be her ending next week.

What makes even less sense is that she attacked the city without even going after Cersei and taking out the palace; it was only after she had set fire to the city that she bothered to go after "Cersei" — without even any real knowledge if Cersei was still in the city. If anything, after taking out the scorpions and opening the gates, she should have directly attacked the Red Keep — she had very little reason to bother with the city.