Towards the end of Samara’s Loyalty mission: The Ardat Yakshi, you’ll meet up in Morinth’s apartment, and Morinth will seduce you.
Choice: Kill Morinth or Samara
Once you’re in the apartment, and Morinth is seducing you, on the third dialogue option, you’ll have to pass an extremely high Paragon/Renegade (Charm/Intimidate) check to get the choice of killing Morinth or Samara, and it’s best waiting until the point of no return before trying it. Head to our Samara loyalty mission page: The Ardat Yakshi to complete this mission successfully.
If you decide to kill Morinth, Samara will be the one to deal the final blow, resulting in Samara becoming loyal to you, giving her better odds of surviving the Suicide Mission.
If you decide to kill Samara, Morinth will be the one to finish her. After the events, Morinth will disguise herself as Samara, and join you in her place on the Normandy. This will make her loyal to you, giving her better odds of surviving the Suicide Mission.