Death is certainly an ubiquitous theme in the poetry of Dickinson, and Dickinson has perhaps been unjustly pardodied frequently for her rather morbid focus on death in her poetry. However, we need to remember that in her time, a preoccupation with death was perfectly normal due to the high rate of death from diseases such as TB. Infant mortality was high, and it was very normal for every family to have lost at least one child. In particular, death was something of a preoccupation for females, as it was they who were expected to pay visits to the bereaved and comfort those left behind.
What is interesting about Dickinson's presentation on death is the way that she discusses this most impenetrable of human experiences in a way that helps to explain and understand it. In "I heard a fly, buzz" for example, the reality of death is rather ironically compared to the hopes of those watching the speaker die in anticipation of seeing the clouds open and God descend. The last sight of the speaker is of a fly, that rather ironically communicates what the true nature of death really is.
In "Because I could not stop for Death," death is depicted as an unexpected ride in a horse-drawn carriage. In this poem, death is demystified and tamed as death is described not as a terrifying experience but as one which domesticates death. The speaker, after all, expresses surprise at the end of the poem when she realises that the afterlife is her final destination:
Since then--'tis Centuries--and yet
Feels shorter than the Day
I first surmised the Horses' Heads
Were toward Eternity--
The focus on death therefore is not always morbid or terrifying. Rather, in her poetry, Dickinson could be argued to be tryping to explore this experience that lies so beyond the realms of human comprehension and discuss some of its many different facets.