JFK died without having passed his civil rights legislation. Lyndon Johnson, who previously had not shown much if any interest, in civil rights legislation used the death of JFK to secure the votes to pass the Civil Rights Act of 1964. In June of 1963 JFK proposed the legislation but the bill appeared to be dead by late October 1963. Johnson was more experienced in legislative politics than Kennedy and was able to use Kennedy's assassination to push the Civil Rights Act forward.

Without Kennedy's assassination The Civil Rights Act would likely have taken a decade or more to pass.

I wouldn't say that McKinley had unfinished business but he did have a different approach to foreign affairs than his successor Theodore Roosevelt. McKinley was fairly popular and I believe could have won a third term. McKinley's assassination opened the way for Theodore Roosevelt who may not have become such a prominent figure otherwise.

Garfield's assassination really helped along the passage of the Pendleton Act, which made it that so government jobs were awarded based on merit, not political/familial association. And made it illegal to fire employees for political reasons. Garfield was big on improving government efficiency.

James Garfield was supposedly an honest john type of President set to bring integrity and honesty back to politics after the scandal-ridden terms of Ulysses S. Grant and the highly controversial election of Rutherford B. Hayes (and the subsequent compromise of 1877).

However, he accomplished very little before dying from botched post-assassination surgery, becoming one of two presidents to die within the same calendar year as their inauguration (the other being William Henry Harrison).