Joker Movie Spoilers
Everything You Need to Know About Joker if You Refuse to See It in Theatres
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After the whole "who are my real parents" debacle, Penny ends up having a stroke. When she lands in the hospital, Arthur goes to see her and ends up suffocating her with a pillow. It's another step in Arthur's downward spiral into his mental health problems. He eventually admits he stopped taking his meds, and you can see him increasingly losing it.