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.