Are you using the word bad here as that which one doesn't hope or desire for? Like for example saying ice cream is good. If so then we're basically saying the same thing. If not, can you explain what context you are using the word bad? — SonJnana
The point is, it doesn't matter how I am using the word bad. It's no different to asking how I'm using the word 'flat' when saying the earth is flat. I can be wrong about the earth being flat
only because we all broadly agree on the meaning of the word 'flat', but the most important thing about it is this doesn't stop being the case if one person in the world decided that 'flat' means "curved like a ball". The fact that
most of the world agree on the the properties of things that belong in the group "things which are flat", is what allows me to say that someone claiming the earth is flat, is wrong. I compare the earth to the widely agreed upon criteria for membership of the group "things which are flat" and decide that it does not meet these criteria, therefore the person is wrong. It doesn't matter if a small group of people dispute the criteria for membership of the group "things which are flat".
Morality is no different. We have a widely agreed upon group "things which are bad". The edges of that group are very blurred, but this is true of many groups. You think you know what a car is, or a table, but it's not difficult to think of ambiguous object which would be difficult to classify (an amphibious car, a piece of furniture designed to be both sat on and eaten from). This does not in some way invalidate the group "things which are a car/table", we get along quite well with blurred edges to our definitions.
So morality is not some special case, it's exactly the same, we have some things which clearly fall into the group "things which are immoral/bad", like murder of innocents (remember, the fact that one or two people might disagree about the definition of a group does not invalidate the description any more than if I now declared that only green things are 'cars', that doesn't suddenly make the use of the term 'car' ambiguous). We also have blurred edges, like adultery, where there is discussion about whether they fall into the group, but this is resolvable by the second point about definitions,
Group membership criteria are not defined first. No-one ever said "all small modes of personal self-propelled transport with four wheels shall henceforth be called cars", the first thing to be called a car was simple called that, and all subsequent things were compared to it for similarity. If they were similar enough they were called cars, if not they were given another name.
Deciding if a thing belongs in the group "moral behaviour" or not can be approached the same way, is it similar enough to other things in the group to justify inclusion. If it is not, then we can justifiable say someone claiming it to be "moral" is wrong. Murder of innocents clearly does not share the characteristics of other things in the group "moral types of behaviour", it is about as unlike to all those things as anything could get, so someone claiming that the murder of innocents was moral behaviour would clearly be wrong.