Planet killers are pretty easy to catch, though. Large bodies like that reflect more light. They've been mapped out and a planet killer is less that 1 in 12 billion chance in the next 100 years. The gap we have right now is bodies around 80-180M which would impact with the force of a few nuclear bombs, but are not enough to end civilization. It wouldn't be great if one of those hit, but it wouldn't end humanity. That's why DART was so important, it showed that we can give bodies in that range a pretty good shove in a short period of time.

https://neo.ssa.esa.int/risk-list
https://www.smithsonianmag.com/science-nature/the-very-real-effort-to-track-killer-asteroids-and-comets-180979206/