Is Shelley's West Wind in reality a destroyer and a preserver, and is Shelley indulging in self pity in the poem?
The "wild West Wind" is, indeed, a destroyer in this poem: it drives the dead leaves, the "Pestilence-stricken multitudes," toward "their dark wintry bed." The seeds seem to lie like corpses in the grave, cold and low, until the Spring wind eventually comes. However, this driving of the leaves and burying of the seeds must occur if spring is to come and resurrect them, bringing new life and new hope. In other words, the destruction wrought by the West Wind is ultimately necessary in order to preserve all life for a rebirth; thus, the wind does actually preserve life as well as destroy it.
I do not know that I would say that Shelley is wallowing in self-pity. He certainly, at least, is meditating on mortality and aging and perhaps even the loss or stoppage of creativity. Speaking to the wind, he says, "Drive my dead thoughts over the universe / Like wither'd leaves to quicken a new birth!" Thus, while he (or the speaker of the poem, not necessarily Shelley) admits that his thoughts are "dead," he also recognizes that this death is required in order to "quicken a new birth" and come to life again. This, ultimately, sounds more hopeful to me than it does self-pitying.