The act of determining the exact ratio viz: "Bad Karma created by accidental killing":"The same created by intentional killing" will require me to thoroughly study all Scriptures ,which ,unfortunately,i have not done.
However,accidental/unintentional killing is also a bad Karma.
Manu Smriti chapter 3,sloka 68 says:
A householder has 5 slaughter houses viz the hearth,the grinding
stone,the broom,the pestle ,mortar & the water vessel,by using which
he is bound.And in order to expiate these sins the sages recommended
the 5 sacrifices(Pancha yajnas) for the householders.
In all these above mentioned 5 "slaughter houses" accidental or unintentional killing of several insects and microscopic beings are done on a regular basis.As an expiation(prayaschitta) for those sins,the householders are thus required to perform the 5 sacrifices daily(the Pancha Yajnas).
Similarly,Parashara Smriti,Chapter 2,Sloka 8 says:
What a sin a fisherman incurs in the course of one year is incurred by
the ploughman with a iron plough within just one day.
So,the act of accidental killing is also bad Karma .In fact, if we go by the above Parashara Smriti verse, then the karma(of accidental killing)can sometimes be much worse than intentional killing(viz-fisherman killing fishes,which is intentional,but a plough man tilling soil and thereby killing several insects/microbes in it is unintentional or accidental).
The dire consequence of unintentional killings(of as "insignificant" as "mere" eggs of insects!) is well reflected in the following story of Kaurava mother Gandhari:
At the end of the war of Kurukshetra, she is supposed to have asked
Lord Krishna, whom she(Queen Gandhari) blamed for the war and the death of her hundred
sons, especially Duryodhan, as to what had been the reason for such a
According to Lord Krishna, long back, while cooking rice, she had
poured hot water of the boiled rice on the ground outside her kitchen.
This hot water killed all the hundred eggs laid by an insect. This act
of hers had earned the wrath of the mother insect who is supposed to
have cursed her that she too would have to endure the deaths of her
sons, as she had.
Now,the act of killing of the eggs was completely accidental yet the consequences were severe as far as Gandhari was concerned.
And,regarding intentional killing of harmful insects- What can be done?These are unavoidable acts.If a mosquito comes & sucks my blood can i sit back and enjoy?Of course not.
Even the Scriptures declare that Dharma Palana is to be postponed in situations when one's life is endangered.So,i see the cases of (intentional)killing of harmful insects as such unavoidable acts.