What were John Wilkes Booth's motives?

Answer 1

John Wikes Booth assassinated President Lincoln in hopes of disrupting Union military tactics, as well as eliminating the possibility of the "Lincoln dynasty" in the words of Booth. He was concerned (as were many others) that Lincoln's tight association with the military and his tendency to act without or against the consent of congress may lead to a tyrannical takeover.

Answer 2

John Wilkes Booth was worried (as many politicians were at the time) that President Lincoln was going to try to overthrow democracy with the union army. Most people today don't think of it, but it was a legitimate concern at the time as Lincoln had already ordered several actions of the army without congressional approval and suspended the Writ to Heabus Corpus. In Booth's own words, "I do this not out of revenge, but to make it so my children, and children's children will not have to live void of democracy in the Lincoln Dynasty."