Loki laughs at his death because of the profound irony and the ignominious, humiliating end to his life, a fate he recognized was a direct consequence of his own foolish mistakes.
When Loki witnesses his own demise at the hands of Thanos, his laughter is a complex reaction rooted in a moment of stark realization and self-reflection. It's not a laugh of amusement or madness, but rather one born from the sheer immense irony of how his life in that specific timeline culminated.
His life, which he had often sought to control and manipulate, ultimately ended in such a humiliating and inglorious manner. This ignominious conclusion, in his own understanding, was a direct result of his foolish choices and missteps throughout his existence. He had, earlier in the episode, contemplated the idea of "forks in the road" and the "wrong paths taken." His death scene serves as a stark, visual culmination of those very errors, making his laughter an almost self-deprecating acknowledgment of his own undoing.
It is a moment where he grasps the full weight of his past actions and the unexpected, undignified conclusion they led to, finding a grim and almost absurd humor in the entirety of it.
