That's how you know they're hypocrites. Can you have rust if you don't first have iron/steel for air and/or water to oxidize? Can you have death without first having life? Can you have disease without a host? Pollution without air or water to pollute? No, no, no, and no. Just as rust corrupts metal, just as death corrupts life, disease health, and pollution clean air and water, so too does evil corrupt good. What makes this rather peculiar, however, is that people who go through evil themselves are often the ones to believe those atheist myths.
Atheists love to taunt us in response to that argument by claiming that morals were invented by mankind. Wait, what? Aren't there societies in the present and in the past that actually believe evil to be good? In fact, there are and were. The most prominent example of this is the most evil of evil societies that was Nazi Germany. It was a society in which Hitler made all the rules, and the resulting consequences were catastrophic. Not only did this society lead the world into a war that would dwarf Woodrow Wilson's "war to end all wars" by a factor of 10, but it also would carry out a hellish attempt to systematically exterminate entire races of people, which of course failed since the people that the Nazis tried to exterminate still exist today.
When that war ended and Americans and Brits once again came out on top, a series of criminal tribunals for the heinous acts committed by the Nazi officials began. These became known as the Nuremberg Trials. One by one, the Nazi officials were sent to court and charges were pressed against them. However, these trials couldn't have been more difficult. Why? Why didn't they just surrender? Because the Nazis' moral compass wasn't of God, it was of Hitler.
That's where the refutation to Euthyphro's dilemma comes in. Atheists will often claim that the morality of an act is determined by A, the intent, and B, the effect of that act. But guess what? Having been brainwashed by Hitler, these puppet murderers actually believed that their heinous crimes weren't crimes at all. Nowadays, we actually have an international set of laws against crimes against humanity, such as genocide, along with a UN to enforce them. Back then, however, neither the UN nor the international laws that it legislates/executes existed. Therefore, if it weren't for a divine set of standards to hold those evil people accountable to, the Nuremberg Trials would have been futile. In order to get the Nazis to stop believing that the intent to exterminate Jews and the effect of that intent were good and imbue a sense of guilt into them for their wrongdoings, the argument of a "higher" set of laws at those trials had to be brought up, and at the time, no such code existed except for the one in the Bible.
So, wait, if evil corrupts good as I said above, then why doesn't God constantly work to keep restoring the good in this world? In Revelation 22, He will ultimately "stop" evil. For now, however, He's given us, the church, that job (Matthew 28:16-20). Whether or not that commission is fulfilled depends on how we as Christians act in front of other people. Although we are all human (Romans 3:23), and were saved not by what we do but what Jesus did (Ephesians 2:8-10), the only way we are ever able to save others is by practicing what we preach and not being hypocrites. Otherwise, if we say one thing and do another, we end up setting a bad example to the newbies. Bottom line: Until evil is stopped, it's our job as believers to be the light in the world that the world may see who God is through the example that we as believers set.