This is is a very short drable about what was might have been going through Levi's head when he saw that Petra was dead... It's super duper short because I wrote it from my phone, sorry.
What makes her so different to him? She shouldn't be more than just another squad member - but she is.
Captain Levi felt a sharp pain in his chest when he saw her lifeless body smashed against the tree. Her blood was smeared everywhere, it coated the tree, stained the ground, and her clothes were drenched.
This was it.
Petra was really gone. This strange girl who put all her faith in him was dead. He would never see her bright smile again, or hear her bubbly laugh. She always seemed so innocent until it came time to fight, then her true colors showed. She never held back anything and would slay titans left and right along side him. She always followed his orders without question, she seemed to adore him and respect him more than anyone else. He valued her friendship more than he ever let on.
Maybe that's why he feels this terrible sorrow deep inside right now. She will never know how much she actually meant to him. Petra will never know that he actually did love her.
The pain in his chest is almost too much to bare - but he didn't have a choice. He knew she wouldn't want him to mourn her right now. Levi had more important things to deal with, like taking down the monster that did this.
Levi could never get back those precious moments with her, he would never be able to even say goodbye. He had to let the thought of her go now or else risk his own life and many more because of making irrational decisions based on his feelings.
Captain Levi took one last look at Petra and silently said goodbye in his head to her.