One common objection to assertion of the right to migrate is the killing versus letting die distinction. This makes a distinction between:

• Killing, or more generally, directly causing a harm to some other person.
• Letting die, or more generally, failing to confer a benefit to some other person.

Further, the claim is that immigration restrictions are closer to “letting die” (i.e., failing to confer a benefit) than they are to “killing” (i.e., causing a direct harm).

Two types of responses are discussed below. The first assumes stronger moral obligations to strangers. The second, which makes fewer assumptions about moral obligations to strangers, is more convincing.

Utilitarian response: drowning children

If the cost of saving a person’s life is very small, then letting the person die is equivalent to killing that person. This is illustrated by the drowning child parable. This response denies a meaningful distinction between killing and letting die, at least when the cost of not letting die is trivial.

To complete the argument, we would need to argue that immigration restrictions benefit immigrants significantly at very little cost to any other party.

Libertarian response: forcibly preventing people from helping themselves is akin to murder

A less utilitarian response concedes that there is a meaningful distinction between killing (or more generally, causing a harm) and letting die (or more generally, failing to confer a benefit). However, the claim is that immigration restrictions are closer to “killing” than “letting die” because they actively impede other people’s efforts to help themselves (and each other). Governments station guards at the ports of entry to block, with violence if necessary, the entry of people who are trying to escape poverty and improve their life chances.

For more on this line of reasoning, see:

• Starving Marvin, a hypothetical by Michael Huemer that sheds light on the killing-letting die distinction. The relevant passage is quoted below:
  

  Suppose that, through no fault of mine, Marvin is in danger of starvation. He asks me for food. If I refuse to give him food, I thereby fail to confer a benefit on Marvin and, at the same time, allow Marvin to go hungry. If Marvin then starves to death, those who accept the doing/allowing distinction would say that I have not killed Marvin, but merely allowed him to die. And some believe that this is much less wrong than killing, possibly not even wrong at all. But now consider a different case. Suppose that Marvin, again in danger of starvation, plans to walk to the local market to buy some food. In the absence of any outside interference, this plan would succeed—the market is open, and there are people willing to trade food for something that Marvin has. Now suppose that, knowing all this, I actively and forcibly restrain Marvin from reaching the market. As a result, he starves to death. In this situation, I would surely be said to have killed Marvin, or at least done something morally comparable to killing him.

  The actions of the federal government of the United States are more analogous to the case in which I restrain Marvin from reaching the market, than to the case in which I merely decline to provide him with food. The government’s immigration policy is not a merely passive one—the government does not, for example, merely fail to assist people in coming to the United States. Rather, the government hires armed guards to stop people from coming in and to forcibly expel people who are already here. The federal government spends almost $13 billion a year on actively excluding or expelling unauthorized immigrants.The United States is like the market where would-be immigrants could satisfy their needs. There are Americans willing to hire immigrants, to rent them living spaces, and in general to engage in all other kinds of needed interactions with immigrants. My charge is not that the U.S. government fails to give Third World inhabitants what they need. It is that the government actively and coercively prevents many Third World inhabitants from taking a course of action that they otherwise would undertake and that would in fact succeed in enabling them to meet their needs. This is much closer to inflicting a harm than it is to merely allowing a harm to occur.

• John and Julio, a blog post by Bryan Caplan.