The Leviticus verse is much more direct than the sheep pen verse. Leviticus doesn't qualify whether the alien is here legally or illegally.
It seems to require some pretty tortured theology to find Biblical vindication for a round-up.
I don't think there was much of a concept of illegal vs. legal residents of a "state" either-- accept possibly for an invasion. There also wasn't a whole lot of handouts by the "state" that one group of people was paying for, but another group was receiving.
All things being equal, you treat people that weren't born here as you would those that were. All things are no longer equal.
There's nothing tortured about my theology.
"Tortured theology" is what it takes to believe that we are called to let our home be overrun by criminals who run amok and just take what they can, and not protect our own boundaries.
Having a "roundup" is a practical issue, not a moral one. It would be impractical to atttempt to deport twelve million people all at once, as even demagogues keep joyously pointing out, but to deport illegals whenever it can be done, is at least a shot at enforcing the law. But of course, many criminals and criminal sympathizers complain that "the law" is immoral (when they want to break it.)