I think it's pretty clear by now. Either....

a) Billy IS Mildenberger (in which case he's pretty damn old and holds a grudge against Ali).

b) Billy had a man crush on Mildenberger (and was more than just a little miffed when he lost to Ali).

c) Billy was Mildenberger's slut (in which case he'd still be pretty peeved at Ali knocking out his man).

d) Billy is mentally and emotionally disturbed (regardless of a, b, or c..... d is likely true also).