Why Evil Gains So Much Power...

As a general rule, the way any system of organization works is that whenever a problem arises, there will be many different groups of people who propose solutions. If the proposed solution fits within the current rules and regulations that society has accepted as valid and workable, then it becomes part of what we call 'law'. This law defines how things are allowed to be done in society and helps prevent disorder from arising.
However, if a proposed solution does not fit within current laws, then it is most often ignored or rejected. In addition to this, sometimes people try to enact a new law which would make the suggested solution valid and workable, but if that doesn't happen in time before disorder arises from no action being taken on the matter at hand, then there's nothing left for society to do but react.

When it comes to any given problem, there are usually many different solutions proposed by people. Some of these solutions may be truly horrible and inhumane, while others may be truly compassionate and moral. If the evil solution is not blocked or rejected by society on time, then it will become part of what we call 'law'. This law then becomes a valid way for humans to act.

In this manner, evil has gained power in human society. We've all seen it happen many times before. For example, a man brutally kills another person and is not punished for it. The act of killing the other person becomes valid because no one stops him from doing so.
In another example, a man forces himself sexually on a woman and she keeps silent about it. The act of forcing oneself on women is then validated as acceptable because no one punishes the man for doing so.
If you think about it, history is a long list of situations like these. Some of them are even enshrined in our religions, such as the story of Abraham being told by God to sacrifice his son Isaac.