There are two ways to interpret this question, one philosophical and the other historical. In the first sense it was inevitable. Why? Because all things which have happened had to be inevitable or they would not have happened. Once something has taken place it can be seen as inevitable that it would so happen. But since this is an historical site, let's try the second interpretation which is that nothing is inevitable in history until it does happen.
When I taught history to my AP high school students I would pose two essay questions to write on. The first, in October, was at what point did the American Revolution become inevitable, the second, in April, when did the Civil War become so. I was always fascinated by the answers I got, from some pretty bright students.
In case you are wondering most students chose something like the Parliamentary response of the Intolerable Acts, the decision of Parker's Minutemen to assemble on Lexington Green. Some chose something less proximate such as the decision to abandon the policy of Salutary Neglect or the treaty of Paris, 1763 removing the French from North America, a somewhat more remote cause.
But as this is a Civil War site I would argue that none of the remote causes need necessarily have resulted in Southern secession, or even given that fact, would an ensuing war have been inevitable. Lincoln could have traded a fort for a state and let the erring sisters go in peace. Certainly there were remote causes making a rupture between North and South more likely to occur, Wilmot Proviso, Federal fugitive Slave Act, Uncle Tom's Cabin, Kansas Nebraska Act, Dred Scott decision but none of this, even all together was sufficient to spark secession and I would argue that if the provocations to offended feelings never went beyond this, the slights would have amounted to little more than what one experiences in the stands at an Eagles-Cowboys football game (yes, if played in Philly there might be some fatalities).
I do think, however that Harper's Ferry, while not making war inevitable, did create the kind of rancorous atmosphere that started the average Southerner thinking seriously about secession. However, even the election of Lincoln in 1860 did not make civil war inevitable. Lincoln could have died before inauguration of typhoid fever, like his son would, or have been murdered in Baltimore on his way to DC or, as I said above he could have traded a fort for a state. He might have backed the Crittenden compromise. Even his decision to reprovision Ft. Sumter need not have started the war. Had the telegram to South Carolina's governor been worded differently, " I am sending only some food, no reinforcements, until we can work out some mutually agreeable resolution to the problem of the present garrison. Would South Carolina care to purchase the fort and take over its upkeep?" Perhaps not likely, but until Edmund Ruffin actually pulled that lanyard contingency still operated and war was not inevitable, but once that friction primer went off fate decreed otherwise.