Hell hath no fury like a woman scorned

I think the reason Mallika is being made by T&D to do all this is when Armaan eventually marries Jassi, we are not left feelin sorry for Mallika.

It's like in the movies. When the protagonist kills the villian at the end of a movie, they usually show a flash of all the bad things that the villian has done. Some kind of justification what the hero is doing.

Similarly so with Jassi, armaan and mallika