The fact that the characters in 'Akame ga Kill' have their own dreams and hopes that are ultimately crushed is a big reason it's a sad story. Akame herself has been through so much and seen so many of her comrades die. The sacrifices made by the characters, often in vain, create a very melancholy atmosphere throughout the story.
The emotional relationships between the characters play a role in conveying the sadness. The love, friendship, and loyalty among them are constantly being tested and broken. For example, the relationships Tatsumi has with his friends are torn apart by the brutal reality of the world they live in. As these relationships are damaged and destroyed, the sadness of the story becomes more and more evident.
The manga of Akame Ga Kill is filled with intense action, complex character developments, and a story of rebellion against a corrupt empire. There are many battles, plot twists, and emotional moments.
The Akame ga Kill manga ran for quite a while. But the exact number of chapters and volumes might vary depending on how you count and which editions you look at.