The left is used to arguing with appeals to emotions, and tugging at people's sense of alarm works to stir people up. What better way to do this than by comparing Trump to Hitler? The problem is that as this becomes more and more common, and as the comparisons get stretched more and more (comparing the temporary detainment of children to Nazi concentration camps, or Trump's Helsinki performance to Kristallnacht), you have to think some on the left will naturally conclude that if Trump or his policies are considered to be evil, then taking him out (or his supporters) is seen as permissible, or even an imperative.
You're right that it is scary.