In poetry and literature, plants are often used to symbolize life. In fact, thinking about human life as a plant is a conceptual metaphor (see Lakoff and Turner or the link below for an explanation of conceptual metaphor). In short, a conceptual metaphor is one that needs little explanation because it is commonly used and built into our way of thinking. Consider referring to someone who is "green" to mean young or inexperienced, in the same way that an unripe fruit or unflowered plant might be green. We also refer to someone as "cut down in the springtime of youth" in the way that a flower in the springtime might be cut; we sometimes refer to sex, or the beginning of life, as "planting a seed"; we might refer to an elderly person as "withering"; and the place where we are born is sometimes referred to as our "roots." In short, flower or plant imagery is often used to symbolize different stages of life, and with the imagery of death in this poem, we should be especially attuned to thinking about how the flower might represent human life.
In this case, we can tell that the roses are doing well, representing perhaps young to middle age, because this is when flowers bloom; referring to the flowers as "blooms" cements this image. It also presumably gives a smell, again indicative of a healthy plant, as death is smelling the blooms that he pulls off. In this case, pulling off the blooms suggests an early death, and the fact that death is literally killing the roses makes the metaphor apparent. Additionally, the image of all of the now-dead roses around death suggests how indiscriminately death kills, wasting as many flowers as he can and not caring about the love or beauty that roses potentially symbolize. Death lays to waste all which is in his path, regardless of age or beauty.