I thought that entire scene was fantastic and well acted. If I had any issue to pick, it would be that Sylvie wasn't even willing to entertain the idea that he might be telling the truth. I get this derives from her trust issues, but like Loki said, the consequences for making the wrong choice are cataclysmic. At least think about it!
See I don't think Sylvie even cares about the consequences. In much the same way that Loki has always been motivated by jealousy and spite, Sylvie is motivated entirely by revenge. She is out to destroy the people and organization that ruined her life, and she really doesn't care about anything else. If all of reality burns as a consequence of her getting her revenge, she doesn't care.
Now granted, the way she kind of slumps to the floor after stabbing He Who Remains indicates that she did in fact realize the consequences of her actions in the immediate aftermath, but up until she killed him I really don't believe she cared about anything else other than doing the stabbing.
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Besides, marijuana is far more harmful than steroids. - BlitzBomb I headbang to Bruckner.