The North wasn't a monolithic entity, so of of course it wasn't "against slavery" in terms of total unity.
But it's the height of deceit to assert that the abolotionist movement wasn't growing in strength—particularly in the North—in the decades leading up to the Civil War, and it's outright absurd to think that Northern parents would send hundreds of thousands of their young men to die in a Civil War, absent a "righteous cause" to unite them.
By the time the Civil War was well underway, there can be no doubt that the abolition of slavery was that righteous cause, and it's absolutely ridiculous to assert otherwise.
While very few wars are fought over only a single issue or concern, any attempt by Confederate apologists to minimize the importance of maintaining slavery as a Right (in the South), or to dismiss the desire to abolish slavery as a righteous cause (in the North) is patently ludicrous.
As Secretary of State William Seward said when the Emancipation Proclamation was issued: "We show our sympathy with slavery by emancipating slaves where we cannot reach them and holding them in bondage where we can set them free."
If it were a "righteous cause" for which they were fighting, why didn't they free their own slaves to demonstrate that this was indeed the cause for which they were fighting?
The actual cause for which they were fighting appears to be stopping those Southerners from trading with Europe without sending millions of dollars North to Washington DC and New York.