Emily Dickinson can be viewed as a rebel because she challenged many traditional notions that people had at that time. For instance, in her poem "Because I Could Not Stop for Death," Dickinson rejects the social tradition of marriage by suggesting that death is a charming suitor. The poem begins with Death being personified as a man coming to date the speaker. Death takes the speaker in a carriage around town, past schools and fields, and the speaker suggests the encounter is rather pleasant. Some may see the poem as pessimistic -- after all, most people do not desire to die; however, Dickinson manages to portray a rather lovely, albeit tragic, relationship. Dickinson's refusal to conform to social convention and her reclusive, poetic nature are in an of themselves challenges to what society expected of women at that time. She did not become a wife. She stayed in her room and wrote what she desired.
I don't see her as a pessimist. Her writings and viewpoints are much too complex to be simply dismissed as being "pessimistic." She is one of the first thinkers to see society in a different light. She does not arbitrarily discredit society as a bad thing, but rather explores the complexity with what it is like to be perceived as different and how does one navigate the valences of holding a belief system that is contrary to the norm. Citing again poem 49, the idea of bringing out a different conception of death is not pessimistic, as much as seeking to evoke a condition or belief that is different from what had been standard acceptance of death. In this we see her as a rebel, a voice of dissent, and not merely pessimistic. I see her as someone who was actively engaged in questioning reality and seeking to appropriate it through language in a different manner than taking what had been standard acceptance.
When I think of Emily Dickinson she reminds me of a modern day person who would probably be in his/her room all of the time on the internet talking to people online and only dress in black. She was a very odd person. She wore white while most women of that time wore dark colors. She would never greet guests and later in life she stayed in her room and only talked to people through letters. Even her poetry was different from that time period not only the content but the style as well. Many of her poems deal with death. For example in poem 49 she writes about death and begging God for Mercy. She had lost 2 friends at around the time that she wrote the poem so it wasn't that she was being pessimistic she was dealing with her grief through writing. I don't think she is discussing death in a negative manner just as an unavoidable fate.