1 He was in Whitechapel on the nights of the murders, and alone at the right times.
2 When he was out of London or I was with him, there were no murders.
3 He can change his appearance easily.
4 He studied medicine. He could easily cut up a body in the dark.
5 He knows the lanes and yards of Whitechapel well.
6 He can escape from the police because he knows their plans - indeed, he makes their plans.
The arguments against:
1 He spends his life fighting crime.
2 I know my friend. I know he could not do these things.