Based on One Piece Chapters 843, 844, and 845.
This will be a tragedy. You have been warned. Also, Character Death. Again, warnings now.
It should be easy to forgive him and move on, Nami knew that. After everything Luffy went through to get Sanji back in the crew, she instinctively knew that there really was no one else that could be their cook. No one else could come in and take his place. No one else would be as accepted by the crew and Luffy as Sanji was when he cooked for them. And for the most part, it was easily done. She smiled as she watched him take joy in cooking again. She enjoyed watching his relationship with Usopp progress, as the two got closer as friends. This was his home, this was his family, that much was obvious. Yet she couldn't quite put out of her mind that awful moment when she'd thought he betrayed them. Watching him fight Luffy; worse, watching him beat up Luffy while her captain refused to fight back, after he'd long reached his limit… She hadn't felt that helpless in a long time, sitting on the side lines, unable to do anything to help either of the important men her life. She'd hated that feeling and she resented that it was her crewmate, her nakama, that it was Sanji would had given it to her.
She just couldn't quite let the feeling go. It should have been the same, accepting Sanji back into the crew. He had no more betrayed them than Robin or Usopp. He'd been fighting to protect everything important to him, sacrificing himself in the progress, the same way Robin had. He'd declared war on Luffy the same way Usopp had. Something about this time was different, however. The feeling after he came back was off. Was it because it had been so completely unexpected? His betrayal seemed so far out of left court, you wouldn't even think to look, expecting it from the right? She couldn't quite put her finger on it but something seemed wrong, off somehow, and there was an underlying tension that seemed to be growing between the crew.
When would this feeling fade away?
The party dwindled down after a couple days and Jimbe said his goodbyes to his pirate crew. They watched in silence as they sailed away. Well, mostly in silence. Usopp cheered them all on, shouting encouraging words to the brave men and women on the crew. Luffy laughed and waved, wishing them luck. It wasn't until after they were out of ear shot, as they were becoming a smaller and smaller speck on the horizon, that dread crept into her chest, leaving Nami feeling cold inside.
She watched, the whole crew did, as the smile faded from Luffy's lips and the innocent joy settled into a serious light. He didn't look at any of them, just stared out into the distance. Suddenly, she knew exactly where the underlying tension was coming from. It wasn't from her. It wasn't from Sanji.
It was from the one source she would have thought was most unlikely. Luffy turned to face all of them, to address them, and she wondered for the first time how this was effecting him.