You massively underestimate the destruction caused by a 1km asteroid. Remember some scientists estimate the asteroid that killed dinosaurs was as small as 11km. In other words a 1km one may be 1/10th the diameter needed to cause an extinction-level event.

A 1km asteroid will cause massive casualties no matter where it impacts. If it hits the ocean it will cause a tsunami. If it hits a remote land it will eject so much debris in the atmosphere that they will block sunlight around the globe and reduce temperatures by multiple degrees, thus reducing food crop yields and causing worldwide food shortages and famine.