a bad person

[Sappho]

If there is no god, then there is no universal moral law, which means morality can be reduced to a social construct coming from an evolutionary goal to protect the species, but nonetheless adapted to our modern society.

I think this consideration of evolutionary contribution to human morality is far too narrow. There is strong evidence showing other apes displaying empathy and a sense of fairness and justice by which we can reasonable infer humans shared in the development of our species (see Frans de Waal for further information). Careful consideration must be given to what is meant by 'protection of the species'. This in no way can mean that evolution results in a system where the species is protected en masse. These traits serve to protect a much smaller unit of the species, mostly family groupings that are closely related biologically. This does serve to continue the species of course, but the benefits do not extend to the species as a whole. As far as we know, humans are the only species with the ability to even consider the possibility of expanding Singer's expanding circles of moral concern to encompass the entire species. Although we can consider it, it's quite obvious through our collective action that as a species we are nowhere near implementation.

Our most valuable evolved trait is our ability to think abstractly and reason. The above traits can certainly be considered foundational, but when our evolved cognitive abilities are brought to bear on morality, the entire game changes. This is why humans have a much greater capacity of expanding our circles of moral concern. Our circles are no longer simply ever expanding to subsume weaker biological relations evidenced by our demonstrated ability to expand circles around shared geography or ideology.

Why then do we call people bad when they break such a rule, other than they do something which is arbitrairy forbidden by previous generation. 
(for instance look at: cultural differences)

Subjective and arbitrary are not synonymous. Simply because we disagree with a previous generation's rules does not mean they were arbitrary; circumstances regarding the need for a rule may have changed or our circle of concern has changed based on additional consideration are possible reasons for the change. I am by no means arguing that historical rules were right and properly justified even for their time, but I think it's a mistake to consider them arbitrary.

Thank you for the interesting reply.
First I would like to excuse myself for being so vague in my description, but I often prefer that over writing a large text which covers every possible mistake or gap. In my opinion, when people start making the vague things concrete, that's when interesting conversations start.

I was indeed too quick by taking evolution as the only reason for morality, and you correctly nuancated (?) that, also 'protection of species', but I'd like to ask you which other cause you see. By the way, I did not mean that only humans can have morality, or why did you include the apes? If it is in the link, then sorry but I didn't read that due to time shortage.

With arbitrary I meant that the basic rules now are, or so I think, made up by previous generations, undepended whether they were right or not.

I hope I made myself a bit more clear this time.