Modern people transmigrating into a medieval type of world, and they insta-adapt to the situation. And they get used to killing far too quickly. Or try to implement modern ideas into primitive places.
In truth if most people wake up in a different world, they would spend some time freaking the "F" out, it's human nature. Most MC's come from either Japan or China both of which are modern first world nations, and killing people is considered amoral and illegal, so murdering people shouldn't be as easy as in a lot of stories. And lastly modern ideas and polices of our world only work because of the long build up process to make them work, you can't just snap your fingers and make it work in an alien world just because it works here.