The historical record shows that how First Nations peoples in America were treated by Europeans is not a good story. However, it is not a story of Christianity persecuting and killing First Nations people.
Read some history. You'll find that most of the evil done to American First Nations peoples was because of greed. Greed for land. Greed for money (gold, silver, beaver pelts, etc.). Greed for fame and adventure. What does the Bible say about greed? Greed is a sin. It is wrong.
"Such is the fate of all who are greedy for gain. It ends up robbing them of life." - Proverbs 1:19 (NLT)
When you say the "indians" were killed when they refused to become Christian, as a general statement this is fiction. It did not happen. When you say "we also wanted the land to populate america with more saintly folks with white skin" you are pulling "facts" out of the air. Yes, Christianity was at times used as an excuse, but in most cases it was self-centered greed--some European people wanted what the First Nations people had, and some of these people used whatever means they wanted and whatever excuse they wanted to get what they wanted. This is not Christianity.
Yes, there were greedy people who called themselves Christians. But Christianity does not teach greed. Christianity does not support greed. Christianity is opposed to greed. Those who took from the First Nations people were not living their lives in a Christian way. They were not following Christ.
And don't indite all Americans for what some American's did. Some Christians gave all they had, including their lives, to truly help First Nations people (such as the Whitman Mission in what is now SE Washington state).