why was hans hubermann guilt-ridden days after the incident at the march?

Hans is revealed to not be a supporter of Hitler.

Because then Max wouldn't have left for nothing.

Hans Huberman was guilt ridden because he gave an elderly Jewish man a piece of bread and both Hans and the man were whipped for that. This also allowed the Nazis to identify him as a Jewish sympathizer and this was an issue because they were most likely going to come search their house. This put Max in danger which is why Max left. Hans is upset because he blames himself for Max leaving and putting Max in danger of being caught while moving to another location to hide.