The Schlieffen Plan was devised nine years before WWI started. And what justification was there for violating Belgium’s neutrality?
What was the justification for a "Belgium" to begin with?
Germany wanted the railway systems in Belgium to supply their troops as their right wing swept through to sidestep the fortifications and the bulk of the French army in eastern France. When the Belgians refused to grant passage to the Germans they went in anyway. In short, the Belgians were in the way.
It also worthwhile to note that the original plan envisioned violating Dutch neutrality as well but this was eliminated as in a later revision. The German staff figured that they could squeeze the troops through without taking over the Maastricht hook.