I still find a bit strange how some people complain about some aspects in SC story line (which, agree, might have here and there some plotholes and plot induced stupidity and things that doesn’t make sense).
However I don’t see how Kerrigan had betrayied Raynor by reinfesting herself? At that time she thought he is dead, killed by Mengsk, and she just wanted revenge while she had nothing to lose (neither much of a connection left with other humans).
Also she or her zerg killing lots of Mengsk forces is normal strategic war operations to assure her success and to dismantle for good Mengsk base of power as well, eliminating future threats (even so she didn’t fully succeed with that, see later Valerian and Nova troubles with Arcturus loyalists left behind).
Sure, she isn’t the nicest person around most of the time, and she have (mass) murder tendencies when is pushed but some people are really too harsh on her character, sometimes for comical reasons. She might be on top of the scale (except for Amon) but she is far from unique in SC in this regard. Also her more murderous period was as her first incarnation as Queen of Blades when she was really messed up suffering as well Amon subtle influences, getting used with her new zerg transformation and desires for revenge, power and even human feelings (she still kept a soft spot for Raynor who she spared every time she could have getting him killed)