Neither Jalal nor Ruqu knew that Ben was a Vishkanya. When Ruqu asked him to marry Ben, though it had her ulterior motives, she had a point which was agreed by all (including his mother and his ministers) as Ben had been labeled as Vishkanya by his own Wife and who will marry her or employ her now and she had no other alternative but to die. Untill such time, Ben declared her intentions to die out of shame, Ruqu had not thought of getting her married to Jalal. She used that opportunity to kill two birds in one stone that get a servent in the name of Ben and push Jodha away from Agra.
I thought it was for 2 reasons -
1. Because Jalal gifted Sikri new town to Jodha, and he knows how insecure and jealous Ruk is so he first gave her something to protect his ear drums later!
1. For not causing him more guilt by staying alive.
Jalal looked like fool after Ben truth came out, if Ben had done something to Ruk Jalal would have felt even worst cos Jodha almost died saving his life.