Trivia: A few moments after Susan rides the moped over the ramp, the American tourist who asks Susan, "Excuse me, do you know if there's a Popeye's Chicken around here?" is Melissa McCarthy's husband, Ben Falcone.
Trivia: When Susan shows up at De Luca's villa and talks about her love for Fine, after she promises to shave Rayna's head De Luca calls Susan "Miss Havisham," who is a main character in Great Expectations, a Charles Dickens novel.
Trivia: When Susan is walking in the hallway of Hotel Modiere, admiring the environment, behind her a drunk man walks straight into the wall and falls flat on his back. He is played by Paul Feig, the film's director, writer, and producer.
Answer: It's never fully explained in the movie, but it seems more likely that Rayna simply had a lot of enemies and people we're trying to kill her for various motives, but not to steal the nuke. For example, the guy who tried to poison her seems to do it out of spite for Rayna. The steward on the plane seems more intent on kidnapping Rayna to get what she's selling (although the pilot before dying says "Stan and I already sold it...", so he may not be after the bomb). Karen Walker, who was a double agent who sold Rayna the CIA names, is more likely trying to kill Susan to avoid being found out and not Rayna. Although if Karen was trying to kill Rayna, she may have already discovered the bomb's location, through various means.
Bishop73
Right, when in the basement, to make Susan believe him, Fine asks her "Did (or didn't) Karen try to kill you?" And Susan realises he had sniped Karen and her driver to save her from being killed by Karen.