Nerdy Fact #1776 Joe Simon and Jack Kirby were both second generation Jewish immigrants and created the character Captain America to take a stand against the Nazi regime before America entered the war, receiving harsh criticism and death threats for their stance. In addition, the two served in the military and fought real Nazis when the US did join the war.
EDIT: In addition, Kirby did not shy away from physical confrontation with Nazi sympathizers in the US as well.
“On occasion the Timely office would get phone calls and letters from Nazi sympathizers threatening the creators of Captain America. Once, while Jack was in the Timely office, a call came from someone in the lobby. When Kirby answered, the caller threatened Jack with bodily harm if he showed his face. Kirby told the caller he would be right down, but by the time Jack reached street level, there was no one to be found.”
He just genuinely, and rightfully, hated Nazis.
Source: Nerdy Facts