The author, Sara Teasdale, wrote "There will Come Soft Rains" in the aftermath of World War I, which to her seemed to raise the prospect of the complete annihilation of the human race. She herself was a chronic invalid, prone to melancholia. She committed suicide at the age of 48.
In the poem, however, we see the point of view of a narrator. We cannot assume that the narrative voice necessarily expresses the attitudes and feelings of the poet herself.
First, we can assume some degree of sadness at the prospect of humanity destroying itself utterly. The notion of "soft rains" washing away the evidence of war and bloodshed seems inherently sad; most people do not rejoice at the idea of the deaths of millions of people (approximately 18 million people died in World War I).
There is an elegiac tone to the poem, but one that mingles sadness at human death with an appreciation of the great beauty of nature and its resilience.