A different idea about Catherine's memories
Apologies in advance for ignorance and far-fetched interpretations (I haven't read the play, nor seen the film in its entirety in decades, though I just saw the ending scenes on TCM), but I think given her already fragmented mental state, Catherine's memories of Sebastian's death/murder/killing are more a result of her mind's attempt to connect and make sense of a series of disturbing sensory impressions than a totally accurate account of what occurred. Shimmering heat, blinding white light, discordant noise; crowds of unpleasant, clamoring people; Sebastian's ill health and over-reactions to the least occurence; the frantic racing through claustrophobic alleyways ending in unstoppable mob violence, the sight of Sebastian's nude body beaten and broken on the pavement and the wild animal sounds of the frenzied youths........... The mind tries very hard to make sense out of nonsense; I think she was so overwhelmed by this awful series of images that her "story" was the only way she could make it all hang together. Then, of course, this terrifyingly real false memory became all she actually could recall of what happened. The cannabalism is part of this unreality (any mention of blood??), in that it is a greater taboo than homosexuality and therefor conveys a greater sense of the shock Catherine experienced; she was certainly not shocked by Sebastian's homosexuality even if she could not (or would not) articulate it, but seeing someone murdered certainly is a shock, especially for a fragile mind.
The tip-off for me was the several skeleton images in the memory montage. They're too obvious and heavy-handed to be anything other than a clue that this "memory" isn't all that real.