One is that he's just a normal US politician, within the range previously seen, he was elected president via the usual mechanisms and will leave office and be replaced by someone else, again via the usual mechanisms. His policies and actions may (or may not) be more extreme than most previous presidents, and many may disagree vehemently with them, but that's the nature of representative democracy.
The other is that he's a major departure from previous elected presidents, via a combination of his corruption, illegal behavior and openly racist and sexist language. People who see Trump in this way also likely feel that the SCOTUS has aided and abetted him in acts that are plainly unconstitutional (despite overwhelming numbers of decisions by lower courts against him). They fear that his language around elections, and particularly even his raising the possibility that he would run again, mark him as having crossed a line that makes it impossible to consider him an ordinary elected president. Many people who feel this way believe that Trump and others in his administration are openly fascist and are working to bring about a fascist form of government within the USA.
If you hold the second view, then the fact that around half the voting population view him as a "perfectly normal" politician is itself a substantial and terrifying issue.