Marcel is a Hybird who can kill Originals. But Alaric is stronger than the originals and isn't affected by the white oak where as Marcel is.

Is Alaric more powerful than Marcel?

Alaric was not weaker than marcel, I'd say they're pretty equal, Alaric overpower klaus( and rebekah pretty easy) but klaus was still capable to stop him from steak him, and we also saw a fight between klaus and Marcel the beast, where there was not much different... So I'd say Marcel and alaric are equally strong....

Which originals does Alaric kill?

Rebekah, enraged and desolate, decides to kill Alaric by killing Elena and causes them to drive off a bridge. Alaric dies in Damon's arms after Elena drowns, but appears to Jeremy as a ghost to say goodbye.