Why did Robert E. Lee pusue his path?

Answer 1

He went with his state.

Lincoln had offered him the job of General-in-Chief of all the Union armies. Lee said he would accept the job, only if Virginia voted against joining the Confederacy.

But they voted in favour. So Lee became one of the five Union Army generals who resigned to join the Confederates.

Answer 2

He didn't.

He was hoping that his home-state of Virginia would stay loyal, and he would be able to accept Lincoln's offer to become General-in-Chief of all the Union armies.

But Virginia voted Confederate, and Lee declared that he would not draw his sword against Virginia. So he resigned and joined the Confederate army - ending up as General-in-Chief of the Confederacy.

Answer 3

Long story summarized: War crimes, treason, and attempt to destroy the union's sovereignty. (For details checkout wiki related sources.)