I've read Is it immoral for mosquitos to take blood from other living things without their consent? and would like to ask the same question regarding an inter-human situation.
Assume someone needs to make another's life worse (e.g. make him sick or even kill him, although that isn't really making the life worse) in order to reproduce himself. Could it be objectively justified to do so, even if it's a crime?
... even if it is a crime?
Are crimes immoral per definition? I don't think so. A law is made to represent morality, but often fails and is rather a thin shadow of justice itself. Anyway, the question still stands.
No. Kant, for example, says in his second formulation of the Categorial Imperative, that you should
Act in such a way that you treat humanity, whether in your own person or in the person of any other, never merely as a means to an end, but always at the same time as an end.
— Immanuel Kant, Groundwork of the Metaphysic of Morals
When you make someone sick or kill someone, you treat him as a means to an end (you use him), and that's the only way you treat him (i.e. not also as an end). You give in no way something back to the human you hurt as a 'thank you', and therefore Kant would consider such behaviour immoral.
Yes. At the moment, I can think of one philosopher that you could use to argue for this statement (whether he'd think it himself as well, is debatable): Locke. John Locke writes that God has made mankind, and has given mankind several things (like the earth). To respect this act of creation, we have to reproduce ourselves. If you wouldn't know if the one you hurt will get sick or die, it would be arguable to hurt him, as the number of people after that would be greater than or equal to the number before that.
Let's assume it is okay to hurt someone in order to reproduce yourself. Then...
As I stated above, crimes aren't per definition immoral.
Yes. According to Kant: sapere aude, i.e. dare to think! Kant found it very important not to rely on authority (the law) but to reason and do what your reason says you should.
No. When Sokrates was captured and held in prison, his friend Criton came to see him. Criton was a wealthy man and offered Sokrates to get people to get him out of prison. He wouldn't be able to walk free in Athens, but he could stay alive. Sokrates didn't accept the offer. Even if laws aren't good or moral, you cannot ignore them - for the sake of order. According to Plato, it's very important to have the people in a country do as the government says - the 'normal' people can't understand the reasons of the philosopher king, if everyone would do as he pleases there would be chaos.
Generally speaking in modern states the right of violence is taken by the state. This is then imposed on the domestic body by policing and external polities by War. One could say this has been agreed by a social contract.
One could say that then a citizen has no right to resort to violence to enhance their reproductive success (I can't see anyone justifying violence in these terms - imagine being in court for murder and instructing your lawyer in these terms!). But a closer analysis shows that this resort to violence is given to the state as proxy. They are the guardian of violence. Since the state represents the body politic, everyones 'reproductive success' is enhanced.
There is a theory of Just War and Jihad that makes explicit the condition underwhich War is permissible. And of course the same goes for policing.
