These books are written on a period of war and killing any men during war is not a sin. But Brahmins are not soldiers but people who pray to God and advices the Kings.
In the modern era, the Brahmins can be compared to Medics in war zone. They help both our troops and theirs and no soldier will hurt medics.
Killing medics is a big sin now.
Killing Brahmins ...
Mosquitos take your drops of blood and give bite marks, pain and disease.
I think it won't be a sin if you kill it in the second attempt
Make it fly away in the first attempt as a warning, and if it comes again to hurt you, then kill it for your self-defence
According to Devi Bhagavatam it is a sin to kill mosquitoes. The reason is that Almighty has created mosquitoes to be like they are. They drink blood as it's part of their Swadharma.
He who kills a black serpent on whose hood there is the lotus mark,
lives in Sarpa Kunda for as many years as there are hairs on his body
and he is bitten by serpents there and ...