Time Assassins by R. Kyle Hannah
History has always been written by the victor, but in the shadows, history has been manipulated by a an ancient Guild of Time Assassins. Until now. Rick Brewer, assassin's apprentice, is sentenced to death for a crime he did not commit. He escapes to a distant past and, stranded in time, seeks revenge against the Guild by creating instability in the time-line by choosing powerful targets - The Presidents of the United States of America. Reginald Mayweather is a ruthless business tycoon and not one to accept no as an answer, but when a business prospect fails, he demands the help of the Assassin's Guild to eradicate the competition by killing his competitor's ancestors. If successful, it could prove disastrous to the time-line on a global scale. Jason Lassiter joined the Assassin's Guild because he wanted to experience history first hand. Little does he know that his future, and the future of the Guild, rests in his hands.