Ultimately filing last of criminal charges quickest – also most horrific (10)

'most horrific' is the definition.
Both the definition and answer are superlative adjectives. Perhaps you can see an association between them that I don't see?

'ultimately filing last of criminal charges quickest' is the wordplay.
'ultimately' indicates one should take the final letters.
'last of' indicates one should take the final letters.
'charges' means one lot of letters goes inside another (charge can mean to charge into something).
'quickest' becomes 'hastiest' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The last letter of 'filing' is 'g'.
The final letter of 'criminal' is 'l'.
'l' going into 'hastiest' is 'hastliest'.
'g'+'hastliest'='GHASTLIEST'

'also' acts as a link.

Can you help me to learn more?