Ok, newish here, so I'll take a long shot stab at this :)
Are you bacteria/germs ?
I am here,
yet most people don't care,
most people carry on without worrying to much about bacteria.
Sure, we wash our hands and such, but mostly we "don't care".
I kill people,
if you are not careful.
some bacteria can lead to sickness/death if you're not careful
can think of some extreme situations, such as during operations, or spreading of a cold/flu
I can be created,
but it's rather useless.
we can create bacteria and diseases:
(not convinced of the useless part though :( )
If you can see me,
It's because of Him.
if you can see something that smaller, it's likely via the aid of a Microscope or such.
(still not 100% what "Him" is referring, but if we call the Microscope "Him" ... :)