It turns out that the area that the Pilgrims settled had been a large settlement a few years before with productive fields cultivated by Indians, yes. However, when Bradford and his congregation landed there in 1620, the settlement was completely gone and the land was empty--with scattered bones suggesting a holocaust of sorts, not by Pilgrims or other Indians but by, as we know now, epidemic.
Now, one could argue that epidemics that wiped out Indian villages along the coast were brought by Europeans. Probably true. Those would originate with explorers, fishermen, and trappers who frequented the coast beginning shortly after Columbus's discovery of the New World. There is no way that contamination could that have been avoided once the rest of the world knew the land was there, especially when they didn't even realize they were bringing new diseases and germs.
In fact, another recent (if controversial in its conclusions) book called 1491 makes the case that the Americas were heavily populated prior to Columbus's discovery and that the Indians of both North and South America were both genetically and environmentally susceptible to the diseases brought in by newcomers. If so, this was a holocaust waiting to happen.
As it was, the Pilgrims, after their persecution in England, just wanted to be separate, and the empty land near Plymouth Rock fit their purposes. Squanto, they believed, was sent by God to help them. Farther south and a few years earlier, John Smith, far from being racist, wanted to promote alliances with the Indians in Virginia by intermarriage--one epitomized by the marriage of John Rolfe, made wealthy by his discovery of tobacco as a cash crop, and Pocahontas, daughter of the area's most important chieftan.
Yes, later Indians were mistreated, but let's not blame it on William Bradford. His intentions were for the "city on a hill" to enlighten the rest of the world. Some of us would like to believe that legacy still survives.