1. in a wicked, evil, or cruel manner; in a way that is devilish or fiendish
2. in a clever and cunning way, especially in a way intended to cause harm or trouble

Examples

• The villain diabolically plotted to steal the crown jewels.
• She smiled diabolically as her plan came together perfectly.
• The dictator diabolically oppressed the people under his rule.
• He diabolically manipulated his friends to turn against each other.
• The scheme was diabolically clever, designed to trap the unsuspecting investors.
• The character laughed diabolically upon discovering the secret passage.