He saw her Mental World do this through his Mind Hunter Pathway. Then he attacked her mind with the Mind Hunter Pathway and the Killing Pathway combined.
He used the trusty Mental Stab Metac to create powerful mental attacks that stabbed into her mind. The Splitting Edge Metac was also used, and it worked well since this Divine Metac is capable of cutting everything it comes into contact with, be they elemental, physical, or spiritual.
He was giving this attack his all, even though she was already dead. On the other hand, her Mental World didn't resist his attacks. Her Mental World acted as if it didn't see his attacks or saw them but thought they weren't threatening.
To be fair, the laid-back attitude of her Mental World was because it couldn't resist at all. But Loki didn't have mercy just because his target was helpless to resist.
