Another thought about Radiant's objective is that Radiant was meant to protect Shallan from responsibility. Radiant is Shallan's ideal of what a respectable Knight Radiant should be and it is often why Radiant comes out to deal with things that Shallan doesn't want to have to deal with. Part of responsibility is shouldering the weight of your actions. We see Radiant shield Shallan from this in one instance where Radiant saw that Shallan was about to kill Ialai Sadeas and instead of letting her do it, Radiant does it and doesn't tell her. In this case Radiant takes upon herself the responsibility of murdering someone, so Shallan doesn't have to. Just like Veil hid Shallan's memories such as the whole killing her spren, Radiant could recognize that the biggest responsibility that Shallan holds is not that of killing her parents or her spren, but the impact these things may have had on the world. Specifically this could be that Radiant is shielding Shallan from the idea that Shallan had doomed the world by killing her mom, so while she remembers killing her mom, Radiant is still hiding the effects that had on the world. Just a thought about what Radiant's role might be for Shallan even if this whole theory doesn't come to fruition.