I say, without John Brown and his Harper's Ferry Raid, there would be no need for secession. Here you might can say that "There would have been no need for secession without slavery", but not because they were trying to protect the institution but to protect themselves from the institution. Had there been no abolitionist, there would have been no secession. Had there been no tobacco, cotton, sugarcane and rice there would not have been any slaves, or abolitionist and for that matter any English colonies.
Had there not been a sectional differences, violence would not have been necessary to resolve problems.
But this is a blame game that only continues. Mistakes and evils were made by both sides, and neither is free from any shame.