Dill has been a negative influence for a variety of reasons:
1. Dill is more interested in the rumors of Boo Radley than either Jem or Scout. We know this because he would go to the lightpole at the end of the street and look longingly at the Radleys. It was Dill who prompted the dare that made Jem hit the house, and it was Dill who proposed the note from the fishing pole. Dill had the idea of helping Boo Radley out with a trail of lemon drops. Other ways that the children chided Boo Radley during the summers were usually Dill's doing. Think about it, do you know of a scene during the school year when Jem and Scout hunted Boo on their own? No, because it never happened.
2. Dill comes between Jem and Scout. With Dill's influence, the two boys outnumber the girl. This makes Jem treat Scout like the girl she doesn't want to be but actually is. This starts his calling of Scout names like "Angel May" and his acting like she is too scared to do the things that boys do together.