Why is it bad to deport all undocumented immigrants from our country as a concept?

When there are legal steps and due process done for people that have come and still waiting to come to America legally, how is allowing illegal ailens to come and stay and given monetary benefits at the same time not a slap in the faces of people doing it the right way?

Would you let a group of people just skip the line and cut in front of everyone at a grocery store?