Your question concerns the last phrase in 1 Corinthians 13:5.
1 Corinthians 13:4 Charity suffereth long, and is kind; charity envieth not; charity vaunteth not itself, is not puffed up, 5 Doth not behave itself unseemly, seeketh not her own, is not easily provoked, thinketh no evil;
When Christ forgave the sins of the man sick of the palsy, we read in Matthew 9:3-4, "And, behold, certain of the scribes said within themselves, This man blasphemeth. And Jesus knowing their thoughts said, Wherefore think ye evil in your hearts?" Here, they thought evil (bad) thoughts about Jesus Christ. They thought that He blasphemed because he told a man that his sins were forgiven.
But charity, according to our scripture, does not think evil of those who are benefactors of that charity. If you love another with the love of charity, you will not assign evil motives to their actions. Rather, you will seek to find an innocent motive for what they have done. And, when they have done wrong, you will not think of the wrong they have done, but of the care that you have for them.
1 Peter 4:8 states, "And above all things have fervent charity among yourselves: for charity shall cover the multitude of sins." Charity will cover a multitude of sins. That is, you will cover over their sins and accept them despite these sins because of charity. You will not repeat their sin to others. You will not hold their sin against them. Your charity refuses to think evil of them. Proverbs 17:9 states, "He that covereth a transgression seeketh love; but he that repeateth a matter separateth very friends." This is the work of charity.
No wonder then, that according to 2 Peter 1:7, charity is the height of Christian maturity and one step past brotherly kindness. No wonder also that it is called the "bond of perfectness" (Colossians 3:14). And, no wonder that in the trinity of the Christian virtues mentioned in 1 Corinthians 13:13 (faith, hope, charity), it is the greatest of the three. May we all grow in this greatest of Christian graces.