I don’t believe anyone could ever keep a secret like this forever, but I don’t know why someone would put it on Facebook! Unless it was a joke.
This is Marty.
https://nypost.com/2017/09/27/wwe-legend-asks-facebook-if-its-ok-to-have-sex-with-his-daughter/
I do. I believe people are quite capable of concealing a murder they committed, especially if they think it was done as Self Defense or righteous retribution.
This does not make it right in any way, but may rationalize the deed just enough to live with it.
He may written it on FB because of a brain injury that impairs judgement. Other old wrestlers are constantly getting into trouble with things they are saying on social media. I think it is from CTE or the cocaine.