For the action of killing to occur full force there have to be
four factors present:
a) The motivation or intention to kill a sentient being
b) A sentient being that is the object of the action
c) You have to commit the action of killing of that being
d) The being must die as a result of your action
If all four conditions are met the full karma of killing is
incurred. Insects are considered sentient beings so there is the
karma of killing involved in taking their life.
We have to remember SOME killing IS inevitable in the whole scheme
of things. Even if we are vegetarians, the rice we eat, or the
grains or veggies and fruits often have insects/worms on them. They
get killed by pesticides, harvesting, etc. Furthermore, we also
cannot avoid using animal products such as leather shoes, leather
belts and even some furnitures like sofa also made of bull's
leather etc.
In order to be innocent of ANY killing, we would indeed have to
starve ourselves and getting rid of all these daily products. Even
then, we would be commiting killing (of ourselves.)
In Buddha's time, many buddha's disciples attained Arhatship. They
achieved highly spiritual clarivoyance who able to see microscopic
organism with naked eyes within the drinking water. Many arhats
realised there are millions of living organisms within so they
avoid drinking water. As u know we human body cannot survive
without water for a day, thus Buddha prohibited his arhat disciples
from using their special ability magnifying into the water as they
are drinking it.
What is the morale of the story implies? Indeed there are too many
living creatures living around us so certain unintentional killing
is unavoidable.
Talking about absolute abtrain from killing, if we skip our meals
and starve ourselves for every minutes, countless living creatures
such as ring worms and other biological organisms will died within
our body organs.
Isn't it we commit killing acts? Buddha Himself gave discourse
about this truth in some sutras when his disciples questioned about
killing karma.
Hence, our thoughts and intention counts, certain pests at home, we
unintentionally killing it is unavoidable. Killing karma will ripen
if the above 4 conditions fullfilled. 
