The Question and Answer section for Native Son is a great resource to ask questions, find answers, and discuss the novel.
I don't think that the author makes excuses or justifies Bigger's murder of Mary. I'm not sure about the combination of words you mention but it may be that, if accurate, the author was trying to be ironic.
Bigger is just stressed and anxious about the way Jan and Mary are treating him. They are treating him like a friend but Bigger feels they are mocking him.
The author uses third person omniscient narrative which gives the reader access to all the characters and their motivations. The narrative voice can shift depending on which character we are involved with.