SASUKE: "Gato is dead, I killed him yesterday, I guess you no longer have a reason to go after Tazuna right?"
ZABUZA: "Since our client is dead so yes we won't be going after Tazuna."
SASUKE: "I know a lot about you two, Zabuza you are known as the demon of the mist, you attempted to assassinate fourth mizukage Yagura who is also three tails jinchuriki, after failing you fled from your village and become a missing-nin.
And you are Haku, probably the last surviving member of yuki clan. I know about your tragic past, I do sympathize with you Haku, I am the last survivor of Uchiha clan so there are things common between us. Anyway I don't have anything against you two but I hate when strong bullies the weak. I have no problem with you two working as mercenary, and even if you work against leaf village I won't bother you because leaf village is not weak. But this village do not have a single ninja so you two trying to kill Tazuna, makes you two in the strong category bulling the weak Tazuna.
Now that Gato is dead you two won't be going after Tazuna but if I let you go then in future you will again get hired by someone like Gato and weak innocent people will have to suffer. The only reason you two are not dead already is because I sympathize with Haku, now give me a good reason not to kill you."
ZABUZA: "You are strong but we are not weak, right now I'm not at my hundred percent and underestimated you otherwise I wouldn't be caught so easily. So don't get cocky, if you want to kill me then get it over with before I get my full strength back."
SASUKE: "You think I showed you my full strength just now. If I go all out your mizukage and his tailed beast are no match for me. You two never stood a chance against me, if I kill you today or after a week it doesn't make a difference."
HAKU: "What do you want from us? If you are not killing us then you must want us to do something for you."
SASUKE: "I am willing to give you a chance to live, but only one chance. I want you two to stay here and protect the people of this village. They have suffered a lot due to Gato and after Gato's death villagers life may get better but only until another Gato appears. So if you two stay here it will be beneficial for village as well as you two. Zabuza you know that Haku is not cut out for the ninja world, let him stay here and have a normal life. You care about him don't you."
Zabuza did not expect this from Sasuke, he thought that Sasuke would demand something outrageous from them. He was thinking what to do when Sasuke demands secret of his village or something like that. He knew escaping from Sasuke would be very hard so he was thinking of best possible escape route.
ZABUZA: "Why do you care so much about this village? You are an Uchiha so you definitely do not belong to this village."
SASUKE: "Should I only care about people of my own village? If my village is at peace and rest of the world is burning, should I not care? I don't understand that logic. There are innocent people living in this village who deserve happy and peaceful life as much as people of any other village. Everyone has a right to live a peaceful life with their loved ones. Every life has meaning, if a person dies then there will be heart broken parents, orphan children, it does not matter which village he belongs to."
Ninja of the shinobi world have been brainwashed from their childhood to only care about their village. They would do anything in the name of mission, no matter if target is innocent or guilty they will finish them off if they got the mission. Zabuza also have similar mindset and he didn't know how to feel about what Sasuke just told him. He never saw things this way.
SASUKE: "You see life of people of wind country is tough, they don't have enough food due to lack of farming land in desert, and water is also in scarcity. While country of fire has lots of resources. They produce enough food to share with country of wind and still have their storage full. If both countries Damiyos and Kage sit down and have a reasonable discussion then they don't have to go to war, but they won't do that. Country of wind would try to use force to occupy lands of country of fire and country of fire would try to exploit country of wind as much as it could for providing resources.
For ninja of country of fire, they are fighting to defend their territory and for ninja of country of wind, they are fighting for resources so their people can have their basic needs fulfilled. If there were no boundries, only one damiyo and one hidden village then combine country of fire and wind have enough resources and people of both countries would not have to suffer due to war. Or if the leaders of both countries were reasonable enough and have love for their people instead of desire for more power then also people of both countries do not have to suffer by the flames of war.
What I am trying to say is try to see past the boundaries, don't be limited by the thinking that your village forced upon you."
Zabuza was contemplating what Sasuke said, all he was doing was the sake of his village, he needed funds to raise the rebellion against current mizukage. There was a lot of bloodshed happen inside his village due to fourth mizukage, he was planning to stop him and become next mizukage so that he can stop those barbaric academy rules and extermination of kekkei genkai clans. Now Sasuke has presented a different perspective to see the world.