Since you specify Norse mythology, I want to open with the disclaimer - the texts I'm going to be speaking about in this answer, namely the 13th century Prose and Poetic Eddas, are not straightforward reflections of Pre-Christian belief. They are mythological, they are derived from often extremely old and long-running oral traditions, they are fascinating materials and products of their time in their own right, but if you are asking about the beliefs of pre-Christian Norse peoples with regards to Loki, that is a different question than how does the extant mythology treat Loki.

To very briefly explain why that is a different question - we have zero attestations of Loki that can be plausibly dated to before the 10th century poem Haustlöng (only preserved imperfectly in the Prose Edda). While their children, particularly Jörmungandr and Fenrisúlfr, are well attested, Loki thenself is of highly uncertain origins. My personal interpretation is that they are a late Viking Age introduction from Norwegian elite contexts, but there are as many interpretations about Loki's origins as there are scholars who have thought about that topic at all. However, by the 1200s, when the poems are compiled into the Poetic Edda and when Snorri Sturluson compiled the oral traditions circulating in Iceland and Norway into the Prose Edda, a poetry manual, Loki has become a massively central figure to the mythology.

Also, I use they/them pronouns for Loki throughout out of respect to the ways they are non-binary within a Norse conception - in the poem Þrymskviða, when Loki disguises themself as a handmaiden, the text starts to use feminine referents to refer to Loki. Additionally, Loki gives birth up to 3 separate times, which clearly breaks down any binaries of gender and biological sex.

Anyway, on to his portrayal into those sources: By the 1200s, Loki has without a doubt been heavily influenced by the figure of Satan. Richard Cole argues that their allies at Ragnarok, the "Children of Muspell," are influenced by Continental anti-Semitic stereotypes (Cole 2017). Their epithets in the Prose Edda are also generally not.. kind. Snorri lists such complimentary terms as "Father of Fenrir and Jormungandr.... Enemy of gods, Sif's hair-harmer, Maker of Mischief, the cunning god, accuser and tricker of the gods, contriver of Baldr's death" (Trans. Faulkes, 1990). Those are pretty unambiguously bad. But, Loki is also "friend of the gods" and sometimes even "loyal friend" as in Haustlöng stanza 7.

Throughout the mythological corpus of the two eddas, Loki is simultaneously the greatest cause of trouble for the gods and the one who solves the issue. Loki persuades the gods to offer Freyja, the sun and moon to an unnamed jötunn in exchange for a wall around Ásgarðr, but Loki is the one who distracts the horse Svaðilfari sufficiently (by shapeshifting into a mare, which is how they give birth to Sleipnir). Loki is coerced into kidnapping Íðunn, but Loki is coerced into rescuing her, and when the rescue attempt results in the death of the jötunn Þjazi, appeases his daughter Skaði when she assaults the gods in revenge. They kill Ótr, but steals the compensation for that killing from Andvari (thereby spurring the creation of the dragon Fáfnir and the great heroic act of the dragon-slayer Sigurðr). Their actions are perfectly, entirely ambiguous. And this ambiguity appears to make them difficult to deal with in medieval Iceland! Where late prose narratives will often turn Óðinn and Þórr into Christian Demons, Yvonne Bonnetain notes that this doesn't happen to Loki (Bonentain 2006). They refuse to be pinned down to one thing in any dimension, and that, as Dutch religious scholar (and regrettably, nazi) Jan de Vries hinted at with the name of his thorough survey "The Problem of Loki", is the point (de Vries, 1933).

As a final note, I would suggest that Marvel actually nails this ambiguity quite well - Loki starts off superficially extremely evil, but constantly works back and forth across the line of narcissistic asshole and lovable anti-hero, culminating in a significant redemption in the TV series. Marvel's Loki, as much as I can quibble about other things in that portrayal, seems to really nail that core aspect of who the mythological Loki is.

Sources:

Bonnetain, Yvonne, 2006. "Potentialities of Loki" in Old Norse Religion in Long-Term Perspectives.

Cole, Richard, 2017. "Snorri and the Jews" in Old Norse Mythology: Comparative Perspectives

de Vries, Jan, 1933. The Problem of Loki.

Faulkes, Anthony, trans. 1990. Edda.