Robert E. Lee promised he would never turn on his home state. His Home US state, Virginia, was part of The Confederate States which is why he sided with The Confederates even though he opposed Slavery.
There were a few reasons why General Gage sent his British troops to Lexington and Concord in 1775. The British had heard that the colonists were storing gunpowder in Concord. They wanted to move to Concord to capture the gunpowder.