Not sure I understand from your post whether you mean the cat that has been violent was put to sleep or one of the other cats has been violent since the other cat was put to sleep?

If one of the remaining cats is violent and has been since the other one was put to sleep it could perhaps be grieving or as already suggested it may be ill (perhaps have an abscess or similar and just doesn't want to be touched as it is in pain).

If the violent cat was the one put to sleep did anyone manage to examine it to see if there was an underlying cause - could it have been knocked over perhaps and again if it was in pain it definitely would have been aggressive to anyone trying to pick it up or whatever.

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Kaz

For kittens, joy is for the asking and smiles are for the giving