Yun Jhin needed quite a lot of blood to awaken the river fully, the question was where he could get it? Even though the blood of the innocent would awaken the river faster, any type of blood would work but he would need higher quantities of it.
He couldn't sacrifice his disciples yet as they were too few and if word got out no one would come to join the sect anymore as no one wanted to walk into a death trap.
He could give the current disciples the mission of gathering the necessary "ingredients" but people knew about the characteristics of the blood river so they would realize what he wanted to do when his disciples went around to gather people.
So the question was how could he gather an immense amount of people to squeeze them dry and throw them in the river?
And do all of that stealthily.
