He is an illegal immigrant, (and ALLEGEDLY a gang member INAL, but a judge considered that to be a credible source. I don't think he is confirmed beyond reasonable doubt he is, but he is the one who had to prove that he is not dangerous to Americans and the accusation was deemed trustworthy enough),, that much is a fact, there's no getting around it, BUT, ICE violated the 2019 court order to legally shield him from deportation (you need to specific "To El Salvador", he could be deported to other countries, just not there), by still sending him back anyway.
It seems that by the Trump admin refusing to take him back (They are not refusing, the president of El Salvador refuses to give him back, and the US has no claim to him. In the meeting between Trump and Bukele, Trump asked if he wanted to send the guy to the US, but he said no. What should the US do with an illegal immigrant who does not have any claim to the US?) despite breaking the rule of law, this sets a "concerning" precedent that any illegal immigrant cam be deported regardless of their current status given by their local immigration court. Yes, although it is really not that concerning, since it did not happen to a citizen (and I feel for the Kiwi who was confused, the media is really trying to make it look like he was a US citizen), or to an immigrant who didn't have any deportation order.