Are you guys speed readers or something? He figured she was a pureblood vampire from the fact that whoever killed the other vampire completely massacred them with out any trouble. Since she is the one that is still alive it only makes since that if she was a vampire she would be a pureblood one.
@Writerfromanotherstar
Wow you are not a smart person. If humans don't like when we are killed by other humans why the hell would you expect us to be okay with Vampires going around and murdering us?