I suspect that very few people get up in the morning and say, 'Well, it's a new day; let me see how much evil I can do today. Yesterday I was only moderately bad, but today I should really be able to do some harm.' Just putting it in such plain terms makes the very concept of intentional evil seem a bit silly. When we say evil here, we're not talking about common, ordinary individual sinfulness. Lust, avarice, blasphemy, murder; the sins that come from the yetzer hara (evil impulse). As bad as those ordinary sins are, they really do have a limited scope. They can mess up an individual or a family, but they don't bring down entire nations. No, the evil we're going to consider is the stuff that brings misery and death to millions, destroys nations and sends whole peoples into exile. Oddly enough, evil of this magnitude does not come about merely by expanding the scope and intensity of the common sort of human sinfulness.
To get a handle on this, it helps to understand that people are born with two conflicting impulses; the yetzer hara or evil impulse leads us into temptation and the yetzer tov or good impulse leads us to do good. These two impulses are built in and cannot be eradicated. With time and training the most we can hope for is the ability to manage them.
With the amount of discussion it gets from the pulpits of the world, most decent people learn to keep the evil impulse on a pretty short leash. If you murder three or four people, folks catch right on that you're a bad egg and deal with you accordingly. Thus, the amount of evil that you can accomplish by giving the yetzer hara free rein is generally limited to you, your direct victims and those who care about you or them. Societal attention to the evil impulse is fairly consistent across cultures. Steal a sheep anywhere in the world, and the people there will have an established way to deal with you. The response may be drastic or mild, but any functional society has a set of rules that serve to keep the evil impulse corralled.
So if the yetzer hara has limited range, the really large problems in the world must come from the yetzer tov. To say it another way, the road to hell is paved with good intentions.
A moment's reflection on history reveals that this is true. Each of the great 'isms' of the 19th and 20th centuries clothed itself in high sounding phrases designed to appeal to the yetzer tov. Communism's, 'From each according to his ability; to each according to his need,' appeals to our sense of fairness. It also mimics God's instructions to care for the poor the fatherless and the widow. As a conservative estimate, communism has killed nearly 100 million of its own citizens. This death toll includes executions, enforced starvation and deaths due to mass relocations. This number does not include premature deaths due to suicide, depression, poor medication and a lack of basic commodities (shortages are the natural consequence of an inefficient philosophy).
In the United States the War on Poverty, instituted in the '60s, was sold with an appeal to our desire to promote 'social justice' and ensure that no one had to live in poverty or want. The results are quite different than advertised. Poverty has not been diminished and inner city children now grow up without fathers, gang violence is rampent, infrastructure in once great cities has fallen into decay, etc.
Consider the history of the Jewish people during the current exile. In various places at different times a spirit of antisemitism has risen up and lead the gentiles to expel or kill their Jewish neighbors. While individual gentiles in such cases might have been motivated by the evil impulse to covet Jewish property, the thing that elevated each incident from mere banditry to genocide was the belief that they were doing right in the eyes of God. The apotheosis of antisemitism was in early 20th century Germany where intellectuals became convinced that Jews were inferior people who should be eliminated for the good of humanity.
One who is motivated by the evil impulse will eventually reach a point where his appetite is satisfied or his conscience will kick in and he will stop doing evil - at least temporarily. One who is motivated by the good impulse, believing that the evil he does serves some greater good, will not stop. Indeed he will see the harms as indications that he has not gone far enough in the pursuit of his goal and will redouble his efforts. This doubling down in the prosecution of a 'noble end' is what causes the yetzer tov to be so destructive.
All of this goes back to the Garden where we ate of the Tree of the Knowledge of Good and Evil. Eating of the fruit of the Tree changed us, and we are no longer dependent on God to tell us right from wrong. That's what gets us into so much difficulty; we pursue ends that we believe to be good but which are ultimately destructive.
So, if it is our good impulse that leads to such destruction, how do we defend against it? In Parasha Nitzavim Moshe tells Israel how to avoid the evils that flow from the good intentions in human philosophies. Or, more correctly, he tells us how to figure out when some 'good idea' will lead to death and destruction.
"See, I have set before you today life and good, death and evil. If you obey the commandments of the Lord your God that I command you today, by loving the Lord your God, by walking in his ways, and by keeping his commandments and his statutes and his rules, then you shall live and multiply, and the Lord your God will bless you in the land that you are entering to take possession of it. But if your heart turns away, and you will not hear, but are drawn away to worship other gods and serve them, I declare to you today, that you shall surely perish. You shall not live long in the land that you are going over the Jordan to enter and possess. I call heaven and earth to witness against you today, that I have set before you life and death, blessing and curse. Therefore choose life, that you and your offspring may live, loving the Lord your God, obeying his voice and holding fast to him, for he is your life and length of days, that you may dwell in the land that the Lord swore to your fathers, to Abraham, to Isaac, and to Jacob, to give them." (Dt 30.15-20)
In context, Moshe is speaking to the nation and not just to individuals, and he is holding out the possiblility of freedom from national destruction and exile. We know, of course, that Israel was unable to keep what was promised, but that's not because Moshe's instructions were faulty.
So what's the secret? First, love God. It's no surprise that the most destructive 'isms' have either explicitly or implicitly rejected God, exalting instead man, nature or evolution. Consequently their paths have typically been spectacularly evil and mercifully short. Hitler's 'thousand year Reich' didn't last twenty years. The second part of Moshe's secret of success is to obey God's commandments. Here the problems are more subtle than those caused by an outright rejection of God. Indeed, it is on the rocky shore of obedience that Israel eventually wrecked her ship. That same rocky shore awaits us today.
The Torah is complete. It has instructions for civil torts, marital relations, moral purity, the behavior of government officials, etc. In other words, God through Moshe has written down the antidote for eating of the Tree. He has told us what is good and what is evil. He has also told us that if we will listen and obey then things will go well for us as a nation. Unhappily, we still have that ability to form our own moral judgments that we got in the Garden. So we read God's laws and think, 'That's too harsh.' or 'Things are different today.' or 'Jesus did away with that.' And so we substitute our knowledge of good and evil for God's. Once that happens, we are wide open for the destruction that comes from the yetzer tov, the good impulse.