i too thought that raj was lying abt naveen's death. according to flashback naveen was shot dead and the look on his face while dying - it was obvious that it was someone he knew and never expected to shoot him.
there could be three reasons for raj telling that he killed naveen
1. he had orders to shoot him
2. he was always unhappy with the fact that naveen was a better soldier than him - finally when raj got the chance he removed naveen out of his way so that there would not be any competition betn him and naveen. this possibility is less b'coz it makes raj look like a villain. maybe he regretted killing naveen later and when he met naina his guilt increased. he tried to win naina's trust and he did it but as raj himself said that naina's love and trust for naveen was greater. he thought that by winning naina's trust and love he will always have her support in the end. but ultimately in the process he himself ended up falling in love with naina - that is why he is in two minds whether to tell her the truth or not whether to help her in proving naveen was innocent or not. but he knows that if he helps naina he is the one in trouble.
pls watch the new year's eve video closely. when bhargav brings naina to raj's room. when bhargav goes out and raj closes the door and turns to naina he has a truimphant look on his face. it was as if he almost expected naina to confess her love for him but she did not and raj was shocked. shocked b'coz her love for naveen was great and raj realised that no one else was important to naina more than her brother so much so that she refused to fall in love or let anyone come in her way of proving her brother innocent.
3. the third and last possibility is that raj is simply testing naina to see whether she still trusts him or not whether she will accompany him on his mission or not. so he lied to ritu.
so now the question still remains - is raj a liar or not?