Wy did sasuke awaken the mangekyo after killing itachi

[Makoto]

If Sasuke had killed Naruto, he wouldn't have acquired his Mangekyou Sharingan. You have to feel the grief and loss of the person you love and care for. Otherwise, no MS. Sasuke felt the grief and loss of Itachi and had witnessed him dying in front of his own eyes, therefore he acquired his MS after feeling that grief and loss.