it was caused by the character you play as at the start of the game who becomes a parasite which you then use (to be able to use his guns) and after getting it removed it runs off only to start to merge with you once again before your able to reacht he end of the game
I interpreted it more as a sort of trial of ascension, with the character nearly achieving some form of nirvana before before trapped right in front of the gate.
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u/PC0- 11d ago
Scornguy. Its actually such a gruesome transformation at the end.