Read it for yourself. Even knowing that what appears on Wikipedia is part liberal propaganda, what he wrote in the Dred Scott decision was so racist that it was anathema to a large part of the population even considering the mores of the mid-1800's.
Slavery was legal; it was constitutional at the time seven of the nine justices ruled. Whether or not the ruling was the spark that convinced Abraham Lincoln he would have to use the military to violently overthrow the pro-slavery U.S. Constitution is debatable.
We may never know what caused Lincoln to do what it did.