Most of the analysis regarding Mary Rowlandson's book, The Sovereignty and Goodness of God: Being a Narrative of the Captivity and Restoration of Mrs. Mary Rowlandson, views the Indians as instruments of Satan, not God. The author herself, in fact, indicates this many times in the narrative. However, if I understand your question properly, you are asking how the novel shows that God is the one that is using the Indians to in some way chasten his people and the land. Since Mary's story is told after the fact (after she is freed from captivity), she has the advantage of time to look back upon and analyze her captivity, what it all meant and how it fit into God's plan. If one believes that God is in charge of everything, then nothing occurs apart from his knowledge and if the Indians were doing evil, for some reason, God chose to allow this. In fact, in the Bible, when Pharaoh refused to let the Jews go, it states that even God sometimes uses evil people and events for his purposes, in this case, to effect the Exodus and bring the Jews out of Egypt.
As Mary reflects back on her ordeal, she wonders if the Indians have not been used to show the settlers that they are wrong in trying to take too much of the land. Perhaps God is using the Indians to punish the settlers for being prideful. Perhaps the reason that the Indians are a sourge to His people (the Puritans) is part of God's will. Both manking and the earth are under the pain of sin, according to the Bible, and mankind AND the land are said to be "groaning" under the weight of sin, awaiting Christ's return.
God may be using the Indians for the greater good of his people, to cleanse them. Sometimes the cleansing process is painful. Mary compares herself to other sufferers in the Bible, such as Job, so perhaps, she reasons, something is going on in the heavenly realms that she does not know about, like Satan being allowed by God to test Job. In this way, the Indians can be viewed as instruments of God, used to teach them a lesson.
Mary Rowlandson's famous captivity narrative The Narrative of the Captivity and the Restoration of Mrs. Mary Rowlandson (1682) does indeed include her meditatons on how the Native Americans must be a God-sent scourge on His people and land. To be more precise, she sees her experiences in captivity more as a personal trial than as a trial of her people. Read the concluding paragraph of her account for evidence of this focus on her personal situation:
Before I knew what affliction meant, I was ready sometimes to wish for it. When I lived in prosperity, having the comforts of the world about me, my relations by me, my heart cheerful, and taking little care for anything, and yet seeing many, whom I preferred before myself, under many trials and afflictions, in sickness, weakness, poverty, losses, crosses, and cares of the world, I should be sometimes jealous least I should have my portion in this life, and that Scripture would come to my mind, "For whom the Lord loveth he chasteneth, and scourgeth every Son whom he receiveth" (Hebrews 12.6). But now I see the Lord had His time to scourge and chasten me.
Rowlandson struggles to understand how large groups of people whom she considers no better than the sevants of the Devil (she calls them all sorts of names, including "hell-hounds") should prosper on Earth. She concludes that they must exist solely by the permission of God. Her view is grounded in biblical precedent, particular in the story of Job.
After reading the entire text through (it doesn't take very long and is certainly worth it!), you can search the electronic version of the text (follow the link below and use the CTRL-f command to open the "find" field) to search for recurring key terms, such as "Job" or "trial." If you combine a complete reading of the text with a search for meaningful terms and phrases in the electronic version, you can come to develop very insightful and very well supported approaches to this captivity narrative.