Northerners opposed Missouri's entrance as a slave state because slave states would have more senators in Congress than the free states would. It would give the South a bigger voting presence in Congress. This issue was the basic cause of the Civil War.
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Many Abolitionists opposed it, but most Northerners were not Abolitionists. The reason they opposed it was because it could have allowed new slave-states, if the local population voted for it.
They didn't like being turned into unpaid slave-catchers.
Around the early 1800's the Northerners realized how bad slaves really were to keep. A lot of northerners still though it was fine to have slaves.
The Fugitive Slave Law.
Southerners favored the annexation of Texas, but Northerners objected that Texas would add another slave state to the Union.