The deadliest love story of Westeros has come to an end, leaving a vacancy on the Iron Thrones. Jon Snow killed Daenerys shortly after she won the Iron Throne she had spent the entire show fighting for. Seeing the devastation that Daenerys and Drogon brought to King's Landing proved to be the final straw on the dragon's back for the secret Targaryen, who met his lover and aunt in the throne room of the formerly Red Keep — which was looking especially grey thanks to the ash — and took a knife to her gut after sharing one last passionate kiss.
The final episode of Game of Thrones had a lot of questions to answer, but the biggest of all was how Jon and Tyrion would react after seeing their Queen, Daenerys, kill thousands of innocent people in her attempt to take the Iron Throne. Jon Snow seemed like he would stay loyal for a while, declaring that despite her actions, she is still his queen. Tyrion, on the other hand, quickly realized his mistakes, uttering that Varys was right and regretting his support for the Queen of Dragons. The long conversation between Jon and Tyrion in the latter's jail cell seemed to have struck a chord with Jon — in addition to all of the conversations that Arya and Sansa have had with him regarding her — and forced him to realize that the only way to save The Realm was to become a Queenslayer.
It appears that Jon's choice to kill Dany proved to be the right one, as the King's Landing that is rebuilt without Daenerys is one that is not ruled under fire and blood, but instead under the care of King Bran the Broken, with Tyrion as his hand, and a discussion among the King's officials on how to best transport clean water to the city. Jon Snow's choice did, however, have some consequences for him. While the Unsullied wished for his head, Snow averted death by being sentenced to the Night's Watch once again.
Jon Snow has never been much of a killer, mostly being responsible for the deaths of unnamed wildlings and wights. The death of Daenerys is undoubtedly the largest character death that Jon Snow had a role in aside from his own, and makes clear that his "purpose," as Melisandre implied to him upon his resurrection, was to save The Realm from certain destruction at the hands of Daenerys.
At the end of it all, Jon Snow proved to be the most crucial figure to the survival of Westeros, having dedicated much of his time to warning people of the White Walkers and stopping Dany from being able to settle into her role as the Mad Queen. His final scene showed him walking North with wildlings, leaving the carnage of King's Landing behind him. Jon Snow never wanted to rule, and it looks like now he'll never have to — but hopefully his buddy Sam found some room in that Song of Ice and Fire book of his to mention that Jon Snow is the hero that saved Westeros.