Because they literally invaded their territory and took over the area, killed their people, wasted their resources (animals, ect) and were just generally awful to them
Mainly because the English wanted to take over America and enslave the ''Indians" is my guess. But also, the First Thanksgiving might have had something to do with it. It wasn't really that they both had poor relations, it was that the English believed the Native Americans didn't belong on their own land and persecuted them for it.
