What was Mildred's motive for overdosing on tranquilizers and what do her actions say about her character?
Millie was attempting suicide because Montag is an abusive husband to her. Her attempt reflects her lack of options and inability to leave her husband, whom she is dependent upon.
It is not specified whether the OD was accidental, or if Mildred was trying to kill herself. This leaves an ambiguity as to whether Mildred is cognizant of her actions or if she is harboring deeply seated pain.
Mildred didn't intend to overdose; she is an addict. Her addiction speaks to Bradbury's negative opinion of the pharmaceutical industry.
Mildred intentionally didn't take enough pills to kill herself. Her overdose was a plea for Montag's attention, as she feels unloved.
It is unclear what Mildred's intentions were; she is mentally ill and the narrator specifies that she is experiencing a psychotic break and is unaware of her actions.