What secrets did Mr. Hyde keep in the novel Dr. Jekyll and Mr. Hyde that were evil?

In the novel "Dr. Jekyll and Mr. Hyde," Mr. Hyde's evil secrets include his violent and cruel behavior towards others, his indulgence in immoral and depraved activities, and his ability to unleash his dark and malevolent impulses without constraint. Additionally, the true nature of his relationship with Dr. Jekyll is kept hidden, revealing the extent of their unholy partnership in pursuing forbidden experiments.