Lyric discussion by Amerefan28 

Cover art for Scream Thy Last Scream lyrics by Pink Floyd

I don't understand that no one understood that obviously this song is about Syd's own mom and one of his sisters. Syd's dad had died just before he was 16 years old and this must have had a devastating impact on him as this came very suddenly. Syd then left his, now quite old, mom and his younger sister Rosemary alone, for London, (although Rosemary went to study nursing in London some time after); his older siblings had already left the house. Now, the most speculative part and yet, terribly attractive, is that during his infamous weekend breakdown, after which this song was written, he realised, or started to be convinced that his mom killed his dad. This would not only explain the song, but indeed the entire meltdown that ensued and the very deep depresssion Syd went into, to the point where he imagined killing his mom shouting "Scream Thy Last Scream"

[Edit: typo]

Negative
Subjective
Fear
Speculation
Family
Murder
Meltdown
Depression