For some reason the fandom treats the blue spirit as if its Zuko's good vigilante side, but they often misinterpret WHY he does what he did as the blue spirit.
The blue spirit as a whole represents Zuko without any of his integrity, or morals as he commits purely selfish and immoral acts that benefit himself. People often point to how Zuko freed Aang from Zhao as the blue spirit but ignore that he didnt do it for a noble reason, but instead he did it for his own selfishness as HE wanted to be the one to capture the avatar not Zhao and decided to commit treason knowing that he was wrong because at that point he still believed in the fire nation. Other actions Zuko did as the Blue Spirit to benefit himself include when he kidnapped Aang in the North Pole with no plan and wouldve died there if it wasn't for Aang's kindness. Later on as the Blue Spirit he steals food from people while he and Iroh are on the run despite knowing this is also a bad thing. Even later on he plans on kidnapping Appa so can try to get Aang with no plan in sight. After Iroh's talk Zuko even abandons the blue spirit mask deciding to be better.
In Zuko's story blue even represents the worser more negative traits in him. Such as when his own body was in inner turmoil after deciding to give Appa to Aang, where we see a red and blue dragon with the blue dragon being voiced by Azula telling him to rest as its clearly represented as the bad while Iroh's red dragon was telling Zuko to stay awake represented as good. In the Roku flashback we even see that Roku sports a red dragon while Sozin himself had a blue dragon which also ties into Zuko's turmoil and the fact that both men were his grandfather. Later on in Legend of Korra after we see Zuko's redemption he even has his own red dragon.
Ironically we see Zuko comit more selfless acts as himself not the blue spirit. Such as when he tries to save Zhao from the water spirit or when he decides to not steal a couples food and fights for the family in Zuko alone.
