Why is 6 afraid of 7?

Because 7 8 9.

Because 7 has fatherly issues, spanning from his dad who rejected 7 as a child.

6 is delusional, and is hallucinogenic, veiwing 7 as a threat.

7's family was killed by thugs as a child, so he spends the night protecting the mean streets of Gotham.

7 is scary.