Just because there is some weak relation to rationalism doesn't justify guilt-by-association. You could have pointed equally out that most of the Zizians identified as MtF transexuals and vegans, and then blamed the latter groups for being allegedly extreme. Which would be no less absurd.
> She may have done some awful shit
Murdering several people is slightly worse than "awful shit".
The Behind the Bastards podcast works by having the podcaster invite a guest on the show and tell the story of an individual (or movement, like the Zizians) to provide a live reaction. And in the discussion about the Zizians, the light-bulb moment for the guest, the point where they made the connection "oh, now I can see how this story is going to end up with dead people," happens well before Ziz breaks with the Rationalists.
She ultimately breaks with the Rationalists because they don't view animal welfare as important as a priority as she does. But it's from the Rationalists that she picks up on the notion that some people are net negatives to society... and that if you're a net negative to society, then perhaps you're better off dead. It's not that far a leap to go from there to "it's okay for me to kill people if they are a net negative to society [i.e., they disagree with me]."
> But it's from the Rationalists that she picks up on the notion that some people are net negatives to society
That belief has nothing to do specifically with rationalism. (In fact, I think most people believe that some people are net negative for society [otherwise, why prisons?], but there is no indication that this belief would be more prevalent for rationalists.)
> She may have done some awful shit
Murdering several people is slightly worse than "awful shit".