Terrible act making humans at home (6,3)

I believe the answer is:

mortal sin

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'terrible act' is the definition.
'mortal sin' can be an answer for 'act' (mortal sin is a kind of act). I am unsure of the 'terrible' bit.

'humans at home' is the wordplay.
'humans' becomes 'mortals' ('mortal' can be a synonym of 'human').
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'mortals'+'in'='MORTAL SIN'

'making' acts as a link.

Can you help me to learn more?

(Other definitions for mortal sin that I've seen before include "It causes the death of the soul - not venial" , "heinous act" .)

I've seen this clue in The Daily Express.

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