Apart from natural disasters (some of which are partially man-triggered) I think the biggest danger is greed.
It drives out worse of the humanity, and causes so many issues and problems...
How do natural disasters became Earth's greatest threat when we, humans make those natural disasters more dangerous and more stronger. We cut trees, we create pollution from air to noise, yes you have a point that greed is a bigger danger but the biggest of them all are us, Humans. We are the threats of our planet, and we also know that we are the only one who can save it,
Exactly, and saving this beautiful planetary home would require some serious awakening of the consciousness, and assuming the responsibility for our actions and inactions. I strongly believe that there is, in fact, a small step every one of us can take literally every day (and in some cases many times a day) to reduce our negative impact. Because otherwise we are just moving towards the collective suicide, often without even realizing that!