Why did John Booth get involved in the Civil War?

rome paid him, politics

It was a 'civil' war and he was a 'civilian'.

Booth was a Confederate sympathizer, vehement in his denunciation of Lincoln, and strongly opposed to the abolition of slavery in the United States. Booth and a group of co-conspirators originally plotted to kidnap Lincoln but later planned to kill him, Vice President Andrew Johnson, and Secretary of State William H. Seward in a bid to help the Confederacy's cause. Robert E. Lee's Army of Northern Virginia had surrendered four days earlier, but Booth believed that the American Civil War was not yet over because Confederate General Joseph E. Johnston's army was still fighting the Union Army.