The world needs a non-religious and still relevant figure to hate on.
No one remembers him for the wonders he did for Germany in morale, economy, and pride. No one remembers him as the man who led a nation on a near-continental conquest out of sheer charisma. They simply despise him because he lost the war and was the most relevant person to direct hate on at the time.
Anyone who finished Code Geass R2 should be familiar with this concept.
Also, to quote the Dark Knight:
"Because he's the hero that Gotham deserves, but not the one it needs right now...and so we'll hunt him...because he can take it...because he's not a hero...he's a silent guardian, a watchful protector...the Dark Knight"