Why did John Wilkes Booth kill Abraham Lincoln ?
Booth was a Confederate sympathizer from Virginia who happened to live in Union territory, and he was upset that the South had lost the war. He was a pretty well known actor for his time, but couldn't bring himself to enslit in the actual Confederate Army.
He and his band of co-conspirators thought that by assassinating Lincoln and other members of the government and military, he could 1) get revenge against the North, and the man who led them, and 2) give the South another chance to break away while the North was leaderless.
Not the most well thought out plan, but Booth drank a lot too. He also wanted to make the assassination as dramatic as possible, choosing a theater where he had acted numerous times, and jumping on to the stage to yell "Sic Semper Tyranis" at the crowd before he ran out - which means "Ever so be it to Tyrants" in Latin.
As the previous post says, Booth was a supporter of the Confederate cause who could not compel himself to join the Confederate Army. His hatred for Lincoln was based on the fact that he viewed him as a tyrant and felt that he had to be stopped. Some argue that he felt assassination was the perfect way for him to act on his feelings since he had a flair for drama (he was a pretty well known actor after all) but didn't feel it right to join the army to fight against him
He apparently saw Lincoln's efforts to extend the right to vote to freed slaves as the last straw and declared that he would silence the man for good.