What is the overall theme of chasing lincoln's killer?

Planning, Conspiracy and the Unexpected.

Explanation:

Chasing Lincoln’s Killer is a description of a conspiracy for the president to be killed and it also shows how vital historical events are usually the result of both luck and unpredictable circumstances.

For example, we can deduce that Lincoln could have been harmed by John Wilkes Booth Lincoln earlier at the inauguration speech if the president, had Booth taken his opportunity. Thus indicates that planning is important and we should be ready to take opportunities whenever we are presented with them.

In the end though, Booth assassinated the president through some stroke of luck he got as he found out unexpectedly that Lincoln was going to go to Ford's Theater to watch a play which was where he assassinated him.