Even I felt the same when I watched this episode. The worst thing is that nobody made an effort to stop Gopi from killing Radha. When she was about to strangle Gaura, ahem stopped her giving her his mom's kasam. He could have done the same even at that time.
Radha had tortured the Modi family a lot. She had kept Meera away from the family. Gopi was away from the family for 8 long years. She left the house at the state of her 2nd pregnancy. During that time any woman will be needing her loved ones be around her especially her MIL & husband or even her own mother.
Radha was about to kill Gopi & had almost done that and she was about to kill her own daughter. Killing for self defence or for protecting others is always exempted.
But what I didn't like in this scene is that she didn't listen to what Meera has to say. I felt bad for Meera at that time. Usually, Meera does not even retaliate when she is in trouble.