Well, in this kind of fanfiction, Naruto might glare at Sirzechs' power out of jealousy. Maybe Naruto, being used to his own strong powers in his own universe, is suddenly faced with Sirzechs' overwhelming power and it makes him feel a bit insecure. So he glares as a sign of his internal struggle with the new power dynamic he's facing.
In a fanfiction scenario, the glare could also be a result of a misunderstanding. Perhaps Naruto has misjudged Sirzechs' intentions with his power. Sirzechs could be using his power for good, but Naruto, due to lack of information or cultural differences between their worlds, misinterprets it as something malicious. So he glares, but as the story progresses, they might come to understand each other better.