It's hard to say much about a person from just one account of his behavior from someone who clearly doesn’t like that person. From your post it sounds like he has some developmental issues, probably some ADHD symptoms. Why are you scared again? Is he being aggressive either verbally or physically?

If you stay neutral he will keep messing things up, and you two will have to just damage control throughout this group project and will have to be sneaky about what to put and not to put in your paper.

I don't see how confronting him will put you in danger though. No need to be aggressive, just be assertive. You are writing a paper together, which allows subjective opinions on a topic - your and his opinion, no matter how inappropriate you think it is, are of equal value. The main point of writing such subjective papers is to be open to criticism and discussion. If you don't like what he wants to put in you have to discuss things out with him about why that is or isn’t a good idea. If you can't successfully persuade him to not put in his opinion, that means that you don't have enough confidence and clear thinking on your own. 'Negotiating' is not the correct process here.

I would just be assertive about what we will put into the paper will have to be agreed upon by everyone involved. Maybe what you think will be 'offensive' might even be an idea that might make the paper stand out among all the others that are just looking to impress the teacher instead of putting forward original thoughts. Good for you that you have another person on your side and you will probably always win in these discussions due to majority support.