Eren’s laughter after Sasha’s death in Attack On Titan was because of the unbelievable pain he was feeling, which resulted in an explosion of emotion that came out as a cynical laugh.
In episode eight of season four of Attack On Titan, Eren learned about Sasha’s untimely and heart-breaking death on the airship, which surprisingly saw him bursting out into a fit of laughter. However, fans have noted that his laughter was not as sadistic as it seemed at first, as he had only laughed when he heard that her last word was “meat.” His laugh was also not joyous, it was rather strained and heart-breaking, which gave off a feeling that he was tired of the atrocities he was constantly experiencing.
Whilst audience members interpreted his laughter as a result of heartbreak, Conny did not take it the same way, as seen in episode 10. He asked Mikasa, “When Sasha died, what did Eren do? Did he cry? Did he regret his actions? … He laughed. What part of it was funny to him? What part of Sasha dying? Explain it to me, Mikasa. Why was Eren laughing? You know everything about him, right?”
Ultimately, it was determined that Eren’s laughter was not meant to be insensitive, but rather, was a coping mechanism to deal with all that had happened.