It's mostly Picard's "evolved sensibility" and his lament/handwringing about deliberately or accidentally taking life. Kirk wrestled with the death of the Salt Vampire for like five minutes and then said "Well, aint that a shame..." He didn't loose any sleep over the space amoeba and was totally gung ho for killing the horta until he encountered it and discovered that it wasn't actually all that aggressive after all. In TOS, they did. Only to people who have never actually had to lead a team of people from a diverse background towards a common goal. Conflict is inevitable and should be expected; how you deal with conflict is at least as important as how you deal with harmony. Of course they did. They had conflicts, rivalries, arguments and sometimes outright pissing matches. They dealt with them. TNG shows us a version of humanity where all conflicts have been eliminated by virtue of the crew's compulsive desire for comformity. That is, they don't wind up in conflict with one another because they go out of their way to suppress any differences they might have and strive to be as similar as possible to one another.