User blog comment:Paul.rea/Teen Wolf News 040616/@comment-4554525-20160407000756/@comment-4091815-20160407001022

The point where the beast killed the doctors was the first time it "remembered" who it was and transformed without the frequencies the doctors were using to control it.

As to why it killed them and not the others, I'd have to watch again to be sure but I don't think our heroes left themselves open to beheading whereas the Dreads just kind of walked right up to it.