There were rather limited details in both the OPs account and that in The Evening Standard's report. The other tragic, and separate, incident, outside a pub close to Selhurst Park on a match day, The Met Police have subsequently stated they're not linking to the game, as per the post above...
Very sad. RIP.
If it's a separate incident to the one the OP mentions, there was another fatality prior to last nights game outside a nearby pub before kick off - https://www.standard.co.uk/news/crime/south-norwood-stabbing-young-man-knifed-to-death-in-south-london-street-a3782141.html