There is no challenge killing A ranks (even in small groups), and there is no benefit to killing them without waiting for people to show up. Everyone can get the same rewards. Hunts reward collaboration, not competition.

Ninjas mess with other people's hunting for no reason. It's not even selfish, just mean, because there is no additional reward for being a ninja.

I agree people shouldn't get too upset, although it's annoying if you ninja and don't tell people. Then you're wasting people's time hunting just to be a jerk. Some people maintain spreadsheets to let people know where to hunt, so ninjas hurt the community effort.

So, don't be surprised if people don't like ninjas, call them out, or blacklist them.

Instead shout in area chat and wait a bit. You don't even have to be part of hunt linksehlls. Active hunters in the area will see it, notify people, and in a couple minutes people will show up. It's easy to help instead of ninja.