Because lets recap: He was tricked, and used, and hungry, and beaten repeatedly, and treated as worthless and disposable, all by a man fully capable of paying him for his work. He was sent alone to deliver a taunt to a dangerous man. He was seven. He accepting the beating he received without complaint, and was further beaten for asking for the payment he'd been promised. He was whipped, and knocked in front of an ox cart.

Chang Ci'an didn't AIM the cart at his finger.

Chang Ci'an deliberately knocked a seven year old orphan child to the ground in front of an ox cart and ordered the driver to drive over the child.

And the driver did.

He manage to escape with ONLY one crushed finger (and, we can assume, many less permanent injuries). Crushing injuries are incredibly serious, and even one so small can be deadly. An orphaned child can't afford decent medical care. Adult Xue Yang has a SEVERED finger, which means that at best he managed to find a doctor who would amputate it for him, and at worst he cut it off himself. At age seven. Xue Yang is INSANELY lucky to have survived to age 8 after this incident.

This is why Xiao Xingchen's suggestion that Xue Yang should have merely cut off Chang Ci'an's finger, or hand, or arm, is so upsetting to Xue Yang. Yes, the revenge he did take was abhorrent and excessive, but Xingchen totally misses the point of the story (understandable in the moment, everything is happening SO MUCH at that point).

Xue Yang, at age seven, had done nothing wrong. He was a child, he was innocent, and he didn't deserve to be treated like his life was worthless by Chang Ci'an, or the cart driver, or the message recipient, or the restaurant owner, or every observer on the street who did nothing to help. And every person who tells him it's "just a finger" further treats the very real devaluing of and threat to his life as normal and fine.

If he, any time after age 7, had been able to exact revenge by cutting off Chang Ci'an's arm, there is nothing equivalent about that revenge. A severed arm is not a crushing injury, and is less likely to produce life-long chronic pain. Chang Ci'an would have good medical care-- a doctor, medicine, pain killers. His life would not meaningfully be in danger. Chang Ci'an had family (and money) to help him with whatever tasks being one-handed made difficult. Chang Ci'an was an adult, who's already lived a life without pain and disability, and who's prospects will not be impacted. Chang Ci'an was an ADULT and therefore better able to understand what was happening and why. And beyond all of that, Chang Ci'an would DESERVE IT.

To say that Xue Yang did everything he did to the Chang Clan in revenge over "losing his finger" is to betray an utter lack of willingness to empathize with Xue Yang. Even the 7yo version of him.

“Hey, how come you’re always being a girl?” Xue Yang asks A-Qing.

“Because I am a girl? What the fuck else would I be doing?” A-Qing says indignantly. “What’s wrong with being a girl? Are you making fun of me?”