The core of Jiang Cheng's character is built on an immense, corrosive guilt that he can never articulate. He watched his family die and his home burn, believing it was his fault for leading the Wen soldiers back, and that trauma calcifies into every decision he makes afterwards. His driving force isn't really ambition or even justice—it's a desperate, furious need to rebuild something, anything, to prove the sacrifice meant something. That's why he's so harsh with Jin Ling and so bitter toward Wei Wuxian; he's protecting the only family he has left, while also resenting the living reminder of his own failure. The tragic part is his love and his rage are the same fire, and he spends the whole story burning himself up with it.
Honestly, his motivation shifts, but never in a healthy way. Post-Sunshot, it's pure duty and legacy. After the truth about his golden core comes out? It's sheer, wordless devastation. He spent years defining himself by that loss, using it as fuel for his hatred, and then finds out the person he hated most was the reason he could even hold that hatred. He's left with nothing to anchor his identity, which is why his final scenes are so quiet. He's not driven by anything anymore except maybe the ghost of a brother he can never properly forgive or thank.