My vote would go to Alfred Nobel's smokeless powder, Ballistite, invented in 1887. Without it, semi- and fully automatic weapons would not have developed.
Mind you, I enjoy shooting when I visit a brother-in-law of mine who is a gun collector (that 357 packs a punch) But I am fully cognizant that without modern gunpowder, the ability to kill - to murder - at a distance would be greatly diminished.
While I affirm the Church's teachings on legitimate defense and just war, I also affirm Her teachings on Original Sin. And, as we presently live in a milieu that affirms none of these teachings except in isolated pockets of civilization - local parishes of the Catholic Church and then only if the laity are properly catechized - the means to kill our fellow human beings with whom we are at variance at a distance allows for very little room for actual legitimate defense as St. Thomas Aquinas addressed the issue.
Therefore, I will go back even further and de-invent gunpowder altogether, beginning with the Chinese who wasted little time in turning it from a medicine for skin diseases (BLAM! Disease all gone. Please hand this to receptionist.) to military purposes.
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Since we cannot de-invent, I lift up a slightly hopeful literary precedent, Marion Zimmer Bradley's science fiction, The Ages of Chaos. I have not read Zimmer Bradley, but my wife told me long ago of the plot; namely, intergalactic civilization is brought to the brink of total destruction. In a grand move (this is science fiction, remember) the use of all weapons that could kill from a distance is forbidden. Anyone wishing to kill someone else must put their own life at equal risk. Swords, spears, daggers - yes. Crossbows, longbows - no.
So ... what would you de-invent?