What causes plugins to break the admin and render plugin text on the website

What causes plugins to break the admin and render plugin text on the website instead of or in addition to the homepage?

I am trying to find a plugin (or better yet some code) that will list the title of all posts in the category on the first category page. I have tried several plugins and many of them break the admin. I have to delete because I can’t deactivate them.

I am doing this because previous posts are time consuming for the reader and they render 404 codes when using pretty permalinks. Specifically /%category%/%postname%/ with a category base of /.

If anyone knows of a plugin or code to accomplish this that would be great. Otherwise, if someone could suggest why the plugin breaks the admin that would be helpful.

Thank you