I'm not saying they don't deserve some form of due process, but they are not entitled to full on trials that citizens get.
Due process could be as simple as can you prove that you have a legal right to be in this country? If yes, you can stay, if no, then you get deported. He absolutely should have had due process prior to being deported. I am not arguing against that.
From everything I've been able to gather on this story, the issue isn't really whether he should have been deported, it's that there was a legal order preventing him from being deported to the country of El Salvador specifically because a rival gang in the country would kill him for being a member of MS-13.