Maybe Pranay is just generally evil and wants Ishita's money, thats why he's trying to kill her.
But, he's trying to frame Radha as revenge for the slap and all the insult. He was acting nice and sweet and caring and gentle all the time so tha Radha woulden't suspect him.
I know it's a horrible thought and I would hate it if that happened, but what say?
Yuks