Entering Economics Ph D from M Sc Behavioural Science
I certainly don't see a MS in Behavioral Science as a negative, and I don't think any econ PhD program that does research in behavioral economics will either.  The reason you're interested in it is the same reason it applies well to that subfield.  That said:

You may make yourself more competitive in a program that emphasizes behavioral econ, at the opportunity cost of making yourself less competitive in a more traditional econ program, so make sure this is the field you really want to focus on.
You will still need the math.  So if you've studied multivariable calculus, differential equations, and real analysis anyway, then you've probably cleared many of the important bars.  If this MS you're considering would come at the cost of taking those core econ PhD pre-reqs, then that would be a problem.  Not necessarily an insurmountable one, however, if you can figure out how to fit those classes in.