This is my take on what happened after Loki was notified that Frigga had been killed. Just some insight on what might have been going through his mind. There are spoilers so be alert.
"So am I not your mother?" There was a pause, a long pause. He had not planned on her asking that question while on her usual visit. It was not part of his script, not part of his extravagant performance, and definitely not what he expected her to ask. Loki had been set on shrugging everything Frigga would say over his shoulder, paying no mind to her words and pretending as if he didn't care, to keep up his appearance as a silver tongued trickster.
He gazed at her for a short while, averting his eyes momentarily to wrack is brain for a deceiving answer. He found one, eventually, and pondered over it for a split second, before making up his mind. Loki went along with it, as he decided, not caring whether he thought it was true or not, and not minding if it hurt her at all.
"You are not," he answered, his voice a mere whisper. Frigga's expression harbored no fear in showing her obvious hurt, and when she left Loki offered no apology. He was right, he thought, she wasn't his mother, not really, so there was no reason not to have said those things. Sure, she raised him as her own, even made it that he was comfortable in his cell, but none of her blood ran through his veins. It wasn't a lie, Loki decided, he gave her his honest truth. He left it at that, no reason to waste his thoughts over it any longer. However, he still felt as though he needed to say more.
The next time someone appeared in front his cell, Loki was not surprised that it was one the guards. He took a few steps forward, mere inches away from the force of energy that separated his cell from the guard. The man opened his mouth, hesitating for a second, before speaking to Loki in a grieving voice. It came to him, at first, in an incomprehensible manner. Not able to decipher what he had been told. Loki sent the guard away, no more words spoken between the two at that, not even an explanation.
The queen had passed that night. He was told no more than that, but Loki wasn't daft, he knew she had been killed. He was calm at first, purposing to himself that she was no loss to him, but then anger bubble within his stomach, reaching through his body all the way to his hands. Loki released a surge of energy, knocking over the furniture that had been placed in his cell. The next happened in blurs.
He tossed chairs, flipped tables, even ripped the shits from his bed. Loki pulled off his coat, throwing it to the ground and stomping on it for good measure. He tugged off his boots, launching them at the wall, only to leave black scuffs up against the white paint. His pillows had been used as punching bags, their feathers drifting all about the cell and landing atop the rest of his mess. When Loki had kicked a piece of broken furniture he cut his foot, the blood rushing out as tears. He seemed to not have cared, the rage in him still causing destruction to the once perfect room.
It had been an hour, at least, before his anger subsided. Loki panted in soft breaths, his hair tangled about his head. He made it over to a nearby wall, letting his back slide down against it, fatigue washing over him. He brought a hand up to push his hair back, letting it cover his face after the deed was done. He noticed his cheeks were wet, the trail of water leading up to his eyes. He had been crying throughout his whole dispute. Loki didn't care at that point, too tired to scold himself for being so weak.
He sat there for awhile, no thoughts, just sitting. Then, he couldn't control the curiosity of what sort of pain Thor might be going through. Obviously, he thought, the same kind as him, maybe even more so. He imagined his brother after Frigga's death, a mess in secret like he was. How lucky, he thought, that Thor got to see her before she went, before her funeral, before she was gone.
Just one more time, he thought, just one more look. To gaze upon her face and be cradled in her arms, like he had so many years ago. It came to him then, the sudden guilt. She had died with words untrue. Loki had created his ultimate lie, the few words he thought to be harmless. He realized he had been lying for so long that he couldn't decipher truth. They were mixed now, reality and fantasy, the same thing. He wished he could go back in time, never find out that he wasn't Frigga's blood related son. He wanted Thor to be his real brother, Frigga his real mother, and even Odin his real father. That didn't seem too bad.
No, he told himself, never think that way. He'd just end up sulking away his entire lifetime. The God of Mischief was no sob story fanatic. He had said he was burdened with glorious purpose and he meant it. This was all just a bit of grieving, he deserved to do so. Then, that thought whisked itself away. Maybe, he didn't deserve to grieve, considering all the pain he might have put Frigga through during her last moments. It made Loki feel sick, this picture of a such a beautiful being dying with words so hurtful and deceiving. It made him feel as though he was floating through space and time, repeating her last words to him over and over again in his thoughts.
"So am I not your mother?" She had asked him. Of course she was, Loki thought again. Then, why tell such twisted lies, he had to argue? At first, Loki blamed it on Frigga, for tampering with his perfectly rehearsed script. The odds were in his hands, he knew how to wiggle his way out of her ordinary visits. Then, she went off script, into monologue of feelings and affections, needed words she so desperately wanted to hear. In the end, she died with nothing. A barrel of lies, with a bit of cruelty wrapped up in a spider's web.
Loki knew it was just in the spur of the moment, that cruel, twisted lie, but he also knew it was his own mistake. He was always good with lies, even in such serious circumstances. Even when the truth needed to be heard, or if a bit of honesty was wanted. She had stood by his side through all her life's moments with him. When he was doubted, she believed his every word. When he would sneer, she would smile. Frigga so desperately held on to the thought of him still being the young boy she once knew, and he never let her see that side of him for millenniums. It was cruel, it was hurtful, it was so utterly Loki in all his glory. The God of Mischief at his best.
He had created the perfect lie, an impossible outcome in Frigga's little world. "So am I not your mother?" she had asked, and he lied. Yes, he thought to himself, she was his mother indeed. A beautiful woman with a golden heart, kind in all her nature. A virgin Mary with two boys so pure, young, and innocent, and he forever wanted her to be remembered that way. Delicate like a flower, yet fierce like an eagle. She was his mother, from cradle to adulthood, the only affection he had ever truly known. Now, those affections were gone, with lies and cruelty in their wake. Eventually, he decided, lies wouldn't be able to hide his worsening heart any longer, not in this reality.
That's all folks! Hope it was to your liking, maybe even made it into the crevice of your hearts. I loved this movie so much and I couldn't help but notice that the only reason Loki decided to go along with helping Thor was to avenge his mother. Such a sweetie, he is~