Diandra used a piece of paper to light a candle from the gas fire. Then she left that paper burning and went away, hoping that the paper would burn itself out. Upen saw the paper and put it out. Which is when the rest of the house got to know what had happened.
Question is who in their right senses leaves a burning paper unattended? Wasn't Diandra wilfully putting the house and all the inmates to grave danger? She might be crazy but even a mad person will NOT play with fire.
When Ali confronted her, she said that the fire is out - it was out because Upen took prompt action. Diandra did not feel slightest bit guilty about what she had done. When Ali told her that she should not light such things inside the house but out in the garden area, she again went- "I've lived longer in the house, I know what to do".
Later she cleverly deflected the argument from "WHY" to "HOW". The HOW she got punished might've been wrong, the WHY was absolutely right .
Nobody in the house supported Diandra - except Minisha, Soni and Sushant - who'd already been punished. Everybody supported Ali, even Praneet and Pritam - they all only said that he was too aggressive, nobody said he was wrong.