Rebekah tried Elijah’s plan first. She could have easily said “fuck this” and killed Elena right away. She didn’t. She exhausted her other options first, even though she owed Elena nothing. Elena, the one who helped murder her father and her brother. Elena, the one she tried to team up with, and then was daggered as a reward. And yet, Rebekah risked her life and the lives of her family to try to help keep Elena alive. She was willing to run, even though she hates it, so that Elena could live. She tried to convince Niklaus to leave Elena, to leave her in peace and continue on with their lives.
Only after Alaric beat her and burned her brother alive in front of her, did she find herself stuck between two horrible choices. So she went to confront Elena, and Matt drove them off a bridge.
Rebekah could have crushed Elena to death, or drained her. She could have done any amount of horrible things to kill her. Instead, she let her drown, knowing she would just pass out and then die. We know there is an ‘afterlife’, so we know Elena would go off and most likely join all two dozen of her parents.
When Rebekah dies, she most likely will not go off to whatever safe place Elena would. Rebekah goes to the after life for Vampires. And who knows where Niklaus goes, as both wolf and vampire.
Rebekah was as merciful as a terrified, abused teenager on the run from a serial killer could possibly be in that situation. After what Alaric did to Caroline, how can anyone seriously be blaming Rebekah for not wanting to have to be scared to fall asleep every night for the next 50 years?? Yes, she was on the run from Mikael for 1000 years, but Mikael was ultimately her father, and would not do the things to her body that Alaric would enjoy doing. That is a whole new level of terror, and poor Rebekah was being told to do it alone. When even Sage knew that she had a pathological fear of being left alone.
So yes, given the choices, Rebekah made the right one for herself and her remaining brothers.