I think you are looking at this the wrong way. You see, contrary to what it claims, the fragment you know as Regulus is not the true being at all, merely another fragment of Regulus its mind. The fragment of course being anger and hate.
Regulus cannot be considdered a real being at all, at least not any more. Regulus is whatever emotion is the strongest at the time.
All the fragments share the memories of when Regulus its mind was still whole instead of the broken mish-mesh it is now.
What I am trying to say is, just because I also have those memories does not mean I am ro turn into the same hateful being you knows as Regulus. John explains to AEanna.