It's not "shocking" at all to anyone who doesn't superstitiously regard firearms as some kind of magical technology that can be disappeared with equally magical prayers/incantations phrased as legislation.
Per The Daily Beast, Abe was shot "with what looked like a bizarre homemade firearm." A reporter on the scene reported hearing "at least two" shots. A former detective describes the gun as a "box-like device ... wrapped in black tape ... smoke can be seen coming from the muzzle."
If there were multiple shots, it was probably more sophisticated than the single-shot, muzzle-loading firearm that anyone can make (including ammo) from a few dollars' worth of innocuous materials available at the nearest department store. But not necessarily a lot more sophisticated.
Victim disarmament ("gun control") laws only prevent those who are not determined to get/make/have/use a gun from getting, making, having, or using a gun. Anyone who wants to get, make, have, or use a gun can do so and no amount or type of government verbiage will ever change that.