Why? (Depressed Eren X Captain Levi)

Eren was having a war with depression, Eren became depressed after finding out what his comrade thought of him. It triggered him to do the unspeakable and even put a smile on his face to confuse others.

Levi notice something different about Eren but shoved it aside thinking it was nothing but teenager hormones. Levi than regret making the decision after a certain event between life and death.

What was the unspeakable thing that Eren does? And why does Levi care so much? What was the certain event between life and death? Can Levi help Eren? And be able to drag Eren from this hole of darkness?
~~~~~~~~
This story is some what graphic so if you are skittish I recommend not reading.