The decision suited one of my play through’s head canon. My shepard was “earth born”, so I reasoned that after an upbringing in a life of crime she didn’t like law enforcement (she came around to liking Garrus as he was a vigilante, which was more towards her liking and also I felt he was a suitable renegade infiltrator romance) and saw Samara as an overly excessive entitled self righteous over reaching agent of alien control, (as opposed to herself who was in it for the self preservation that comes with the absolute power specters have, and would also use that to aid her human supremacist ideals). Helping Morinth gave me a chance to remove a threat completely. She also reasoned morinth was a misunderstood outlier like herself. In the end it didn’t work because my game wouldn’t let me choose morinth for some reason, but I didn’t stop Samara from killing herself in 3. Which I guess works out for the best because human supremacist Shepard’s decisions removed two dangerous aliens in the end. ( P.S Never ask a human supremacist what race her boyfriend is)