Yes, it's exactly about self-control and repression. 'Simmer' literally describes a pan of water kept just under boiling point. The water doesn't actually reach boiling point so it never spills over the side of the pan. Simmering with emotion is metaphorically the same thing - the emotion is under control, kept just below the surface. For example, if somebody simmers with anger, it will be clear from their facial expression that they are angry but they won't shout or throw things.

It just describes how she experienced the feeling of hatred. Imagine a pot of water simmering. The water is extremely hot, a fraction below boiling point. There's steam rising. There are bubbles. That's how she experienced her hatred. You couldn't experience it any more intensely than that.

Se había sintiendo odio que variaba de intensidad-- a veces fuerte y a otras menos así.