Logic is not a strong point when it comes to BT shows and KYM is no exception but what could be the reason for Neev's guilty behaviour?
Why did he make that impassioned speech to what looked like a dummy, about how he cannot stand to see Prachi married to Milind and he will finish off anyone who came between him and his love? Why did he try to hide the knife wound? Why did he not take the petrol can to the police nor report the car registration number which Ranvir gave? Why did he give Suki those strange looks when he realised she had witnessed his outburst? Would he not have realised that she would have confided her suspicions to Prachi? Why then did he not explain himself to Prachi? There are far too many unanswered questions and may be there is more than meets the eye. I'll put down some of the explanations that struck me.
According to Milsi, Neev seems to have hired a detective who was trying to find out who was behind Milsi's life. The detective seems to have told him that the killer is some one close to Milsi.
Neev now wants to trap the killer and eliminate him so that he is no longer a threat to Milsi's life. So as part of this plan he decides to act as the decoy, that is divert people's attention on himself. He wants people in SM to think that he is the danger man. This will let the real killer think he is safe to act as he chooses and hopefully make mistakes in the process, which will then identify him to Neev who can then proceed to eliminate him.
He may have bought petrol the day before the MA wedding to stop the wedding by starting off a fire. Surprisingly Milsi himself stopped the wedding, so he did not have to use the fire. However, the killer had also thought of the same plan but not to stop the wedding but to kill Milsi. What Prachi found was the can he had bought and not the one used by the killer. That is why he hid it since it would implicate him.
Coming to the car registration number, he may have realised that the number was fictitious when he tried to trace it and hence did not bother to hand it over to the police. He may have had his suspicions about Ranvir but may not have have solid proof against him.
What about his wounds? Maybe he had hired those goons to come to SM inorder to tackle the goons to be sent by the killer on that night. Somehow the killer seems to have wisened up to his plans and may have called off the goons in the last minute. Neev, who is not aware that the killer had changed his plan, in the meantime was waiting for the goons to come. Only his goons turned up not the killer's. Thinking Milsi's life is in danger he goes to his bedroom to protect him but in the ensuing confusion he gets wounded by Milsi. He spoke to his goons (who were there basically to safeguard Milsi and attack the real goons) outside the SM which Suki had overhead and thought were plans to harm Milsi. Since he does not want anyone in SM know what he was up to he tries to hide his wounds.
Coming to the last incident. Neev comes to know that Milsi's life is in danger if he remains in Mumbai. So he stages the whole union leader thing, knowing how Milsi will react and insists that he should accompany him to Pune. Only Prachi's plans to stop Milsi was getting in the way. The uncharacteristic way in which Neev tried to get Milsi to Pune even after Prachi's fall showed how desperate he was to save Milsi's life. He knew that Prachi would be taken care by her family but Milsi was in greater danger if he stayed on.
Knowing that Milsi's life is in danger he follows him like a shadow which was unfortunately misunderstood by Prachi. By the time he could regain his bearing after being spotted by Prachi, Milsi had already left the party and Neev was forced to follow him and finally saves him by taking the bullet hit himself.
Why in all this did he not confide in Prachi? Maybe he thought that if she was aware of the risk he was putting himself to inorder to save Milsi, she may not allow him to do his job. Neev may have realised that in the process of saving Milsi, he would either be killed or may kill the killer. If he kille then the rest of his life will be in jail. he was prepared for even this but was sure Prachi would never allow him to do this.
So at the end of the day Neev's strange behaviour is solely because he loved Prachi so much that he was willing to risk his life to save the life of her love. As they say there can be no greater sacrifice than the sacrifice of life, Neev's life has been one long journey of love and sacrifice for the one he loved.
Sorry about the length of this post.