Actually, to me, to classify something as evil in the context of Buddhism... I find it very strange. There is nothing in this world, even samsara that can be considered evil.
Evil is something that is based on a person's perspective. So for example I was brought up in a regular society that tells me stealing is bad, while my friend was brought up in a society that tells him stealing is good... then our perspective of what's evil will be different.
What I believe is not about 'evil' but I believe in Karma, for that is the universal truth that is equal to all regardless of what his or her believes that is controlled and contaminated by delusions anyway.
So the question here if we remain silent when it comes to something that can potentially hurt someone is not the best character a person should have... But is it considered evil? No, I dont think so... If it is not evil, what is it then? It means we are excessively, overly SELFISH that we do not care what happens around us or even to people closely associated with us causing us to remain silent just to preserve our own self (which wouldn't last long anyway).
I do not believe keeping silent is 'EVIL', but I believe if the action or result that come from us keeping silent is harmful, then we too are the cause of it, we allowed it to happen because we did not say anything against it. And yes... karma will catch up that when we are in that situation, we will face the similar or worst situation.
Therefore, if we are Buddhist, then we will practice first to open our heart and make everything that harms others OUR BUSINESS... then we can speak up with wisdom that will not damage anyone. After all, sometimes even with our best intentions to speak up to prevent something bad from happening, due to lack of wisdom, we end up making things worst.