The difference between these two sets of things is that the Missouri Compromise was the sort of law that reduced tensions while the Kansas-Nebraska Act and the Dred Scott decision were factors that served to increase tensions.

The basic tension during this time was over whether slavery would spread.  The North was, for the most part, no interested in abolishing slavery where it existed.  However, the North was interested in preventing slavery from spreading.  The South, of course, wanted to spread slavery so as to preserve or even increase the political power of the slave states. 

The Missouri Compromise was a way of accommodating these opposing desires.  This compromise (along with the Compromise of 1850) specified which territories would become slave states and which would become free states.  These compromises reduced tensions because they ended the disputes over the issue.

By contrast, the Kansas-Nebraska Act and the Dred Scott decision tore up these compromise and made conflict reappear.  In the Kansas-Nebraska Act, Congress overrode part of the Missouri Compromise and opened these areas to “popular sovereignty” even though they had been designated as free under the Missouri Compromise.  The Dred Scott decision took things a step further, declaring the Congress could not legislate on the issue of slavery in the territories.  This made compromise impossible and meant that every territory could turn into “Bleeding Kansas.”

Thus, the Missouri Compromise (along with the Compromise of 1850) tried to reduce tensions while the Kansas-Nebraska Act and the Dred Scott decision increased those tensions.