Really?

He was in the middle of a transformation. Of course he fell down. And who knows if the bullet had magic in it.

I think we need to consider that either:
A. Seth still has a body of a human, so despite being a descendant of a dragon/having magic, he's still as vulnerable as mortals.
B. The gun has a special bullet (like vampires can only be killed if stabbed with a silver in their heart or superman's kryptonite) that can specifically kill dragons & its descendants.

But, unless we see the next chapter, we'll never really know ┑(￣Д ￣)┍

The next chapter is on manga buddy. But it isn’t the official chapter.