Origin of the expression "to do someone in"

I'm sure the origin can be argued, but this almost certainly came from the east end of London.

When you say 'do him' or 'he did him' it means to beat someone up. So 'do him in' goes further.

Other common cockney phrases are 'Kick his head in' or 'punch his face in' so it only makes sense.

'In' being invasive, meaning entering inside, which is generally how you kill someone.

Coming from London myself it seems perfectly normal to to use 'in'.