This is defintiely a possibility and strong idea for motive, a jilted lover, but respectfully I wouldn’t say this is the “most commonly believed motive”, per say, at least from what I’ve seen here, but I haven’t read every post or polled everyone following this case, but I think a lot of/some people also think a strong possibility is a perp who simply had the desire to kill, a person who feels rejected/inadequate, serial killer type, as I’ve seen that mentioned a few times. All theories are on the table of course, and I’m not narrowed down to one theory.
I would think if the perp was someone close to the victims, they may have been identified by now as the perp, but who knows, maybe they covered their tracks well.