As for the car, I think it's possible that one of Shikha's co workers could be behind that. The guy who really likes her in her office, maybe he has some negative shades too, but we aren't suspecting him. We should expect the unexpected.
What if Sameer actually is a sweet guy? I do think that Sameer loves/is infatuated with Shikha, but what prompts him to kill her, I don't know. Is he the actual killer? I don't know that either.