Is there different kinds of slave holders in Douglass's Narrative? Are they all depicted cruel (including Mrs. Auld) ?

There are different kinds of slave holders....... not all are depicted as cruel. You may want to look through gradesaver's character list.....

http://www.gradesaver.com/narrative-of-the-life-of-frederick-douglass-an-american-slave-written-by-himself/study-guide/character-list/

Gradesaver also has a thorough and detailed study guide for each chapter, as well as a detailed look at the themes.