Yes Raj killed Navin..because navin ASKED him to do so
When Navin had eloped the whole army was behind him...Rajveer caught him in the jungle...and Navin asked Raj to kill him...
Because he could no longer take those court marshals and that torture and so Raj shot him...
It was such an emotional scene...I had goosebumps...Imagine how diffiicult it was for Raj...Superb scene but very very sad