So Trump the eeeeevil deporter was doing it right his first term. How do you know he's not doing it right now? There are a little over a million people here illegally who are already under deportation orders.

Because we are talking about specific instances where he's not doing it right. This isn't a general argument that every deportation the Administration is doing is bad - it's that the Administration is doing specific things to specific people that not only appear to violate the law, but are based on extremely disturbing legal arguments.

The Administration isn't claiming that the people it shipped off to El Salvador without going to an immigration judge were all under prior deportation orders - they're claiming that the 1798 Act gives them the power to deport anyone without due process and without any judicial review. They didn't arrest and detain Mahmoud Khalil because he had a prior deportation order - they're claiming that the Sec of State can revoke lawful permanent resident status if the green card holder engages in speech he condemns. Etc.

It absolutely is possible to deport a large number of people without breaking the law - which in no way is a rebuttal against specific arguments that particular actions the Administration is taking are unlawful. If anything, it heightens them. If the dudes that were shipped off to El Salvador were, in fact, already subject to deportation orders then it would have been child's play for the Administration to just deport them under regular process. That they dusted off the 1798 Act and tried to use that as the basis for doing so should make you very suspicious of any claim that these dudes all had prior deportation orders.

Part of why this is happening is that there aren't enough people here illegally with prior deportation orders for the Administration's numbers to get where they want. A fair number of those folks aren't deportable at all - some now have legal status that trumps the deportation order (like DACA), some are in jail and won't be deportable until they're released, and many more are just whereabouts unknown. During the campaign, it was easy enough to claim that there was some master list of a million or more easily deportable aliens that could just be scooped up and easily put on a plane - and that they only reason that wasn't happening was because of Perfidious Democrats - but now that Trump has all his people in charge of ICE they're finding that exceedingly difficult to deliver. The numbers aren't where the Administration wants them to be, because there wasn't as much low-hanging fruit as the Administration claimed there would be.

You can deport an Obama-level number of people while following the law, but Trump wants to deport many times more folks than that. You can't do that with just folks that have prior deportation orders - you're going to have to go after more 'conventional' folks that are entitled to a hearing.