Original: Lincoln's assassin, actor and Confederate sympathizer John Wilkes Booth, had also plotted with fellow conspirators, Lewis Powell and George Atzerodt to kill William H. Seward (then Secretary of State) and Vice President Andrew Johnson respectively.
Simplified: Lincoln's killer, John Wilkes Booth, was an actor and Confederate sympathizer who had plotted with other men to kill the Secretary of State, William Seward and the Vice President Andrew Johnson.