What was one key event from the 1850s that escalated tensions between the North and South? How did the push for western expansion impact this event?

One event from the 1850s that heightened tensions between the North and the South was the raid by the abolitionist John Brown on the federal armory at Harpers Ferry, which was then in the state of Virginia.  This raid helped increase tensions because the North and South reacted to it in different ways.
In this raid, Brown and his followers took control of the federal armory.  Their plan, such was it was, was to give out weapons to the slaves who, they were sure, would flock to them.  These slaves would then carry out an armed rebellion.  Brown and his people were defeated and Brown was later executed. 
The “argument” on the part of the North (or at least on the part of many Northerners) was that John Brown was a hero.  Northerners saw Brown as a martyr for his cause.  They felt that he was a courageous man who had stood up for what he believed in.  This infuriated the South.  Their “argument” was that Brown was a killer who was breaking the law in an attempt to get the slaves to rise up and kill the people of the South.  They hated the idea that the North would lionize a man who wanted to cause the massacre of Southern whites.  In this way, the debate between the two sides was not really about the raid, but about how people should perceive Brown.
Westward expansion did not have a great impact on this event. It happened in Virginia, not in any western area.  However, expansion did have one impact.  Westward expansion brought about the Kansas-Nebraska Act and the violence of “Bleeding Kansas.”  John Brown was part of that violence.  He helped lead anti-slavery fighters in Kansas and was involved in an incident now known as the “Pottawatomie Massacre,” in which five pro-slavery men were killed in cold blood.  Although Brown was already an abolitionist, we could argue that westward expansion helped to radicalize him and make him more likely to carry out such an extreme act as his raid on Harpers Ferry.
https://www.ushistory.org/us/32c.asp