Is a person held accountable for the bad thoughts that come to mind concerning others, and do these thoughts fall under the category of ill-will?

There is no accountability for mere thoughts as long as they do not translate into words or actions, based on the hadith: "Indeed, Allah has forgiven my Ummah for whatever occurs to them in their hearts, as long as they do not act upon it or speak of it." One must divert their mind from such thoughts and utilize their intellectual energy in acquiring religious knowledge, contemplating the texts of the two revelations (Qur'an and Sunnah), and reflecting upon the signs of Allah and the affairs of the Afterlife.