There you go. The mudslingers would have us all believe that the North was such a nobel enterprise that it fought the evil slavers to the South to end slavery. So it wasn't to end slavery at all then. It was to prevent slavery expasion to the West. And why would the North want to prevent such expansion? Purely humanitarian reasons?
In order to contain slavery to the areas where it existed. Keep slavery out of the territories and each new state created from them would be free. Each free state had two anti-slavery senators. New free states meant anti-slavery congressmen which would dilute the south's disproportionately high congressional representation. All would keep slavery penned up so that it would eventually die. So yeah, the Republicans looked forward to the day that slavery would die. They were just realistic about it.
So why did the southerners want to open the territories to slavery?