It is the second hybrid Jeremy's killed, but the first had a very clear purpose: hybrid is blocking door, Alaric needs to get to the hospital, kill hybrid, now get Alaric to ER. This one... less clear. Killing the guy who risked his own life by disobeying Klaus and letting Stefan free Elena, just so Elena will stop freaking out? That is morally gray, and in the "charcoal" gray area rather than, say, "dove" gray. So the ring theory could fit neatly into that behavior change.
The writing to bring about Elena/Damon is annoying. Where did Special Snowflake's love for Stefan go? Did it just die? Did it turn sibling like in an instant because her lady bits are a throbbing madly for Damon? Is this some sort of underhanded shade directed at Damon/Elena that she can only be with him as a vampire and "dark",but not be able to do that when she was human?
I swear that part of this is actually Stefan having a hard time dating Elena as a vampire. He tried to keep her human, then he tried to keep her vamp-vegan, and at each step it's been clear that Elena is very aware of how invested Stefan is in her NOT being a blood-drinking full-blown vampire. It's been driving her to Damon at every turn, and Stefan's been encouraging it because he doesn't want to (or can't) get involved with that side of vampire life. He's leaping at this cure thing; even Damon is openly commenting on how Stefan might only be able to be with Elena if she's human. Stefan hates everything in him that is vampiric, and has spent about a century convincing himself that those instincts are evil and wrong while glorifying human instincts. So when his lovely human girlfriend who represents all of the things he wants to be suddenly turns into a woman indulging in all the traits he hates about himself... Basically, I think "you like Damon" was one hell of a convenient cover.
I think Damon knows that Stefan's having this particular issue, and if Elena hasn't figured it out specifically, she's certainly picking up on Stefan's disappointment that she can't live a vegan, human life.