entroumi: Probably not, but Outlast Whistleblower maybe? ;) It features seeing killer playing with corpse.
Do you remember anything else than what you listed already? Maybe killer's outfit?
IIRC the "doctor" handles a dead corpse. In "Outlast Whistleblower" he actively kills people while the PC witnesses that, respectively the doctor "handles" the PC itself in "Outlast WB".
There is a scene where the PC moves at night through the landscape with having high rocks - about 10m / 33 feet high - on the right side, where that person seemingly watches the PC from a distance. IIRC the graphics was black/white/greenish in that scene (not sure). I remember the part with the person watching because the guy who made the walkthrough went dilirious, being scared, saying "OMG there is a guy on the mountain watching me" (in german language) several times. On the way further he did see / the walkthrough video showed that figure several times again along the way the PC took outside.
Well,
maybe there is a scene taking place in a tunnel of the metro (underground train) You and a few other persons moved in the tunnels between stations, I can not remember if you fled. But I rather think that that was in an other games walkthrough I watched (roughly) at that same time ... or it was part of "my" game"...
The cloth of the guy which took the body apart was - again I really am not sure - white, maybe like doctors wear in hospitals or like butchers do (skirting). Sure about this: He was white (caucasian), rather tall or average, not overweight neither skinny.
And that guy/killer (not a real doctor) does not talk to the PC but rather speaks "into the room".
BTW: I did not find the scene I described above (watcher on the hill) in "The Park". Maybe the player in the walkthrough of "The Patk" didin't notice the person on the rocks because that is not a cut scene and it's not so much leaping to the eye. Or the atmosphere is just very similar and "The Park" was not even "partly" the game I am looking for.
I'll try to remeber more and post it later if there is something...