Hello and sorry for my bad English.
I play Halo 5 on Xbox and PC. But on PC I can’t aim. Whatever the key I choose … My Keybord work, but not for aim … But the Key is work on a vehicle.
And ob the mouse is the same problem …
I’m alone with this ?
Just use the Xbox controller to play on pc then
I play on PC for the Keybord/Mouse not controller 
My friend haven’t this problem, why ?
Please 343 look this subject and search with me :’(
try to contact 343