For me; the real question is, Why didn't Lara ask Harry specific questions about his plan to keep Thomas safe and hidden. I mean Harry had to have told Lara something like, "I can put Thomas in a kind of stasis that will both protect him from tracking spells and his own hunger, but everything I need to do it is on an island in Lake Michigan" All Lara would have had to have said was something like, "Why there?" and "How does it work?"
In theory, Harry could have lied to Lara if she had asked one or both of the above two questions, but that's not really Harry's style and Lara would know that. So in this situation, Lara charging Harry with a blade is odd; and maybe even somewhat out of character, but what is even more odd is Lara not doing her due diligence.