Indeed Jinn exist and they do obey or disobey Allah depending on how they exercise their choices like us human beings. However, the whole talk on bad ruh or evil spirit cannot be substantiated or traced through the teachings of Quran and Sunnah.
No ruh of dead person will come and harm after his or her departure from this world.
Witch or evil do exist and can harm but our belief is that whoever obey Allah and strive to purifies his or her souls will not be harmed.
2. Jinns can harm like the way human beings harm.
Refrence is chapter
Quran 6:112
Understanding of the whole chapter Jinn helps one to understand the world of jinns