Why did the Missouri Compromise make abolitionists angry?

The Missouri Compromise angered abolitionists because it allowed the expansion of slavery into new territories. By admitting Missouri as a slave state while balancing it with the admission of Maine as a free state, the compromise effectively legitimized and extended the institution of slavery. Abolitionists viewed this as a betrayal of the principles of freedom and equality, as it reinforced the idea that slavery could coexist with free states. Furthermore, it set a precedent for the future expansion of slavery, which many abolitionists vehemently opposed.