The United States has created a lot of screening and processes to make sure that the people who will enter this country would not be a source of harm. To give amnesty to undocumented immigrants can give the possibility of the country to be in danger because they have avoided the proper process. These processes make sure that the people who will enter and choose to stay in the country would be fit physically and financially. Skipping these processes could let people of burden to be in the society.
Some illegal immigrants do not pay taxes, some of them live off of it. They receive food stubs, health charity and government help from taxes that they do not pay for. They pay for nothing yet they get rewarded for it. Since they do not pay taxes, some of them earn more under the table than minimum wage employees who do their part to help the society by paying taxes.
Entering and staying in a foreign country that does not allow it is illegal. Anything illegal is wrong. Whether you do it for a good cause, if it is illegal, it is a crime. Anybody who manages to make a crime is a criminal. You would not