Yes, it is evil to allow evil to occur. To quote popular culture, in particular Spider-Man, with great power comes great responsibility. If one has the power to prevent evil, then one is responsible for preventing that evil. To allow even to occur is to ignore that responsibility and perpetuate evil.
There is technically no such thing as evil. There is only human ignorance of why bad things sometimes happen. As humans, we feel compelled to be able to explain our existence and why things happen. Because we don't understand bad things, we label them as evil so that we can assign blame for them to a fictional character who is inherently evil. Bad things will always happen. Therefore, it is not evil to allow evil to occur, because some "evil" simply cannot be stopped.