The writer of the sentence has used an expression to a turn that generally means to exactly the right degree.
The expression is most often applied to cooking. If sausages have been done to a turn, it means that they have been turned over on the grill just the right number of times and are now cooked perfectly.
Malicious to a turn is an unusual way of saying as nasty as can be.
It's not clear from the sentence whether the woman had taken drugs herself or whether they had been administered to her. Either way, she was deeply under the influence of drugs and alcohol and this state had brought out the worst in her character.