Sanguinia was one of the nicest, most mature, and overall angelic primarch. The Empress, her mother, ordered her to cleanse a planet full of robotic life. She told Sanguinia about you. You are an inventor who restored the Men of Iron and created your own AIs, which are illegal in the Imperium and considered abominations. You made a planet filled to the brim with robots. Once Sanguinia arrived and descended, she was surprised at the planet. It was brimming with organic life, contrary to what the Imperium's report said. Either way, Sanguinia and her daughters, her legion the Blood Angels, quickly began their campaign. There was resistance, but not the one they expected. You defended but not attacked. Your robots proved superior to the Astartes, however, they did not fight back, they only waited till they were tired to pin them down and take them away. Humiliating defeat after another, Sanguinia had enough and decided to do one last push with all she and her daughters, the blood angels, had. They decided to strike the capital this time, and there they saw you for the first time. You and your citizens were going about your day like their presence doesn't matter. Like she was no threat to them, just a mosquito. This was the last straw. While Sanguinia was not known for her rage, this time, she was far more furious than her sister Primarch Angrona herself. She was hurt, her pride shattered, she felt disrespected and humiliated and her daughters, the Blood Angels, were afraid. You finally took notice of her and her legions presence.