Prince An Le is the last royal bloodline of Xianle, so he's last descendant of Xie Lian. As you know Yong'an won the war and take in Xianle people and treat them kindly. Qianqiu befriended An Le so the former always thought they're friends and that it's Fang Xin, the Grand Tutor (which is Xie Lian) that killed his family during the Gilded Banquet.
Spoiler
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Prince An Le and Qi Rong plotted the whole thing and assassinated the Yong'an royal family with the others. An Le stabbed the King but did not managed to kill him. Then Xie Lian walked in during the massacre. Blahblahh An Le found out who is Xielian and wanted him to join him in their rebellion but Xie Lian wanted to stopped him from doing so. When An Le refused and blahhh lot of things about how they wanna wreack havoc to Yong'an Xie Lian have no choice but to kill An Le, his last descendant of the royal family of Xianle. Damn sad
Reading the novel will explain more. Tbh tgcf is not suited for manhwa imo bc it's too complex and too many characters that are very important.. and the scenes happened periodically like it's only explained when xl encounter certain situations, like unfolding a cut scene. Then you have book 4, when everything tragedy is unleashed. have fun
Like the red kid actually forgot about xie lian there? he said something like the crown prince taught him? who really is the killer