I think it actually made healing more difficult as I believe he would have tried his best to be fair and just in administration, appointments, policy, ect. He also had vast experience that may have been able to use after his presidency was up. The loss of presidential office does not mean the loss of ability to positively effect America.
His death may have even poisoned the Union against the reunion some. It also may have undone much of what was held together in the administration. It did do wonders to his already quite credible reputation, martyrdom has that kind of effect.
Why the question?
Lincoln's assassination HEAL union? More like the other way around! His assassination did the opposite of healing, as he made everybody have to be somber and depressed over him, losing lots of tempo in the recovery of the union. So, no, Lincoln did not have to be assassinated--if he wasn't, he probably would have helped heal the union even faster!
First off, Abraham Lincoln accomplished many things in his time as president. The idea that his assassination was "necessary" has virtually no supporting evidence, so I would say that of course it wasn't necessary. It seems that every great president has had an assassination attempt, and although good presidents can be categorized as being ones who end up assassinated, it's surely not a requisite.
Killing in the name of religion or politics is terrorism, and that is not acceptable in any situation. It makes me sad when these things happen. I am sorry, but I have to get around the word minimum, so: I said hip hop, hippy to the hip hop, you don't stop, rock it to the bang bang boogey, up jump the beat.