One concern was that the compromise would upset the balance of power between free and slave states in the Senate. With the addition of Missouri as a slave state and Maine as a free state, the Senate would be evenly divided between the two sides. This could make it difficult to pass legislation related to slavery, as both sides would have an equal say in the matter.
Another concern was that the Missouri Compromise would not effectively prevent the spread of slavery into new territories. The compromise prohibited slavery north of the 36°30´ parallel, but it allowed it to continue in the territories south of that line. This meant that slavery could still expand into new areas acquired by the United States, such as Texas and the Southwest.
Finally, some white people simply did not want to see slavery abolished at all. They believed that slavery was a necessary part of the Southern economy, and they were willing to fight to protect it. These individuals viewed the Missouri Compromise as a threat to their way of life, and they feared that it would eventually lead to the emancipation of all slaves in the United States.
In addition to these specific concerns, there was also a general sense of anxiety and uncertainty among some white people about the future of the country. The United States was a young nation, and it was still struggling to define its identity and its role in the world. The Missouri Compromise represented a major turning point in the nation's history, and it was not clear what the future would hold. This sense of uncertainty and unease contributed to the resentment and fear that many white people felt toward the compromise.