“It is [the] certainty that they possess the truth that makes men cruel.”

Simplified Meaning:

When people are absolutely sure they are right and believe they know the truth, they can become mean and harsh. This strong belief in their own rightness can lead them to treat others badly who don't agree with them. For example, if someone is convinced that their way of doing a job is the only correct way, they might get angry or insult others who have different ideas or methods, even if those ideas might also work well. In history, this kind of thinking has caused a lot of conflicts. Think of the ways different groups have clashed over religious beliefs. Each side was so sure that their own beliefs were the one true way that they ended up fighting wars, hurting and even killing people who thought differently. To avoid this, it's good to be confident but also open-minded. Understand that it's possible to be wrong, or that there might be many ways to look at things. This can help us be kinder and more understanding towards others.