Hanjime Isayama is going to kill Levi in Attack on Titan S2 because Levi is to famous and he wants Eren (Maincharacter) on the Focus.

But

1. Levi os not coming up that often, there's a really big focus on Eren.
2. Some Fans ship Eren and Levi together. IT'S no like they don't like him. For a lot of fans they belong together, with or without relationship.
3. Levi is important for Eren. Eren is looking up to him, since his childhood and he would loose his self confidence if even humanitys strongest dies. Eren is just like 15 years old, how should he save the whole world if even Levi dies? There's a lot of Pressure on Eren because for a lot of people he's world's last hope. Levi helps him to deal with that. Levi supports him and Eren is trusting him completly.
4. Without Levi, Eren and Mikasa wouldn't be even still alive. He protects them and saved their lifes. If Levi dies, the focused Maincharacter could be in danger fastly, maybe because he's unfocused and psychologically confused because of Levis death.
5. Levi is humanitys strongest soldier. Who could be able to kill him? Even Eren was able to attack the collosal titan so there's really no one on Levis level.
6. Levi is a virgin. He can't die as a virgin.