The main theme that we've gotten to work with for most of the actual supersoldier-type serial killers we've seen so far is that they have some kind of code to killing. There's definitely supposed to be a discrepancy between the random serial killers Kuroko was offing early in the series and the people we are led to believe are truly strong, "professional" killers like Kuroko, Conrad, or Reiko.
Conrad, in particular, brings this up when meeting the man in charge of the Murder Party. He asks him what his "Killing Rule" is and when the man says he doesn't know what he's talking about he becomes enraged. It's obvious that someone like Kuroko (and Conrad) likely follow some preset rule or code to their killings. Ties in well with Kuroko's initial interaction where she says she won't kill Hinako because there's no value in it.
Kuroko is a monster with no remorse for the people she kills but that's likely because she's just following her code. Conrad is also unlikely bothered by his (presumed) killings and we can assume the strongest characters that we haven't run into yet are similar. I imagine one of the reasons for Ringo's father asking Kuroko to take Ringo under her wing (and Kuroko accepting) is to give Ringo a focus for her talents and make her a stronger killer with a specific goal and code instead of being just a serial killer. Mindwiped Terumi might be similar to Reiko, as he was obviously talented but I don't think he had a real focus.
We don't exactly know Kuroko's killing rule for sure but it's likely some combination of "people who deserve to die" / "what killings will benefit and consolidate my situation going further" although the "hundreds" of kills she racked up prior to the series start really muddy the waters here. The only one we know anything about is that she killed Hinako's parents, who were apparently evil people.