Roku: I offer you this wisdom, Aang. You must be decisive.
Kyoshi: I offer you this wisdom, Aang. Only justice will bring peace.
Kuruk: Aang, you must actively shape your own destiny, and the destiny of the World.
Yangchen: Here is my wisdom for you, selfless duty calls you to sacrifice your own spiritual needs and do whatever it takes to protect the World.
That's what the audience and Aang infers but wisdom is neutral. They are telling him that it his call to make, that whatever choice he does take, it has to be decisive, just, bring about change and ensure the world's safety.
Roku:
Aang: "Everyone expects me to take the firelord's life, but I don't think I can do that."
Roku: "... I tried to be disciplined and show restraints [i.e. like you are considering doing] but it backfired. ... Sozen took advantage of my restraint and mercy [i.e. my mercy in not ending his life]. If I had been more decisive and acted sooner... I offer you this wisdom, Aang. You must be decisive."
Proper interpretation: If the only option available to you is to kill him, then you must be decisive and do so.
Kyoshi:
Kyoshi: "...I stopped [Chin] and the world entered a great era of peace."
Aang: "But you didn't really kill Chin..."
Kyoshi: "Personally I don't really see the difference [i.e. killing him directly or indirectly makes no difference to me.] But I assure you, I would have done whatever it takes... [i.e. if killing him directly was needed, then I would have done so.] ... Only justice will bring piece. [i.e. only stopping Ozai, whatever it takes, including killing him, will bring peace]"
Aang: "I knew I shouldn't have talked to Kyoshi [because she's casually advising that I be willing to kill Ozai.]"
Proper interpretation: You have to be willing to do whatever it takes to bring peace, even if that means having to kill Ozai.
Kuruk
I agree he doesn't tell Aang to kill Ozai. He just laments having been a lazy Avatar. (Korra's much better than this guy.)
Yangchen
Aang: "These past Avatars: They keep telling me I'm going to have to do it. [Aang, at least, is understanding that he's being told he's going to have to kill Ozai.] ... Maybe an Air Nomad avatar will understand where I'm coming from. [i.e. Maybe she will give me an option in line with my value for not killing people.] Avatar Yangchen, the monks always taught me that all life is sacred..."
Yangchen: "Yes, all life is sacred... Avatar Aang, I know that you're a gentle spirit, and the monks have taught you well, but this isn't about you. This is about the world. [i.e. the needs of the world outweigh your desire to avoid taking a life]... your sole duty is to the world. Here is my wisdom for you, selfless duty calls you to sacrifice your own spiritual needs and do whatever it takes to protect the World. [i.e. you must lay down your value for not taking a life, and be willing to kill in order to protect the world.]"
Proper Interpretation: "I know you don't want to kill, but you must if that's what it will take to save the world."
Further, Aang's conclusion (that he "has to kill the fire lord") is in line with this interpretation.