I think if Dany hadn't made that speech and had just been satisfied with defeating Cersei and now ruling Westeros Jon wouldn't have decided to kill her. It wasn't enough though and she had to go for "world domination". It was her lust for more conquering that was her undoing. As Bronn said "Kill a thousand they make you a king" Lots of rulers have bloody pasts but are remembered for their benevolence like Odin in Thor Ragnarok but she wanted to keep going like Hela. Jon killing her wasn't punishment for King's Landing it was preventing more death. It's like after Hiroshima and Nagasaki, instead of peace the US went "Now it's your turn Russia! And you Germany, even though Hitler's dead and all you countries in Europe and Asia who want to mess with us are going to get nuked!