Loki is the foremost deity I work with, so I know what he is like. Fenrir is not the same as Loki, he is not inherently "bad" and his mannerisms have nothing to do from the fact that he's Loki's son. I would say his behavior and negative connotation comes from the drama surrounding Tyr's betrayal and the Aesir locking him away. Loki is not inherently bad either, and can be a very positive helpful deity.
Having said that, I acknowledge that neither Loki nor Fenrir are deities of "love and light". Both of them have a very dark side to them. Fenrir, especially, is not a deity people can easily connect to and work with. But they're not all bad. It's not a black and white picture. Same with the Jotunfolk.
Loki definitely isn't a deity of light. Nor is Fenrir. I can't say Loki is inherently bad. He just has his ways of things and they can be mischievous. And really, that could just be circumstantial and may or may not be with the intention of causing trouble. It is in his nature for mischief. But bad or good is a matter of perspective. Take what we get from the myths. There's times where things he suggests go sour, but even with the troubles, it gets better. Chaos and balance.
But we can also see Loki from the side of truth. Especially the truth that is not wanted to be heard. We see this especially in Lokasenna. This is a lesson of Loki: no matter what we speak, be it truth or lie, be honest with yourself. When you speak truth, speak the unwanted truth. The truth that other's don't want to hear.
Fenrir is not alike his father in inherently being "bad." His trauma led him to excessive anger and hate, but also deep pain and sorrows. Odin had him tied down. Tyr who was good to him tricked him into being tied down. But despite this deception, Tyr still tended to Fenrir in his captivity. Personally, if i went through great traumatic experiences as he did, I'd be a whirlpool of like emotions to that of Fenrir. He's definitely not one any one can work with. Nor do i think just anyone should work with him.
There's a great lesson we can learn from Fenrir. It has to do with the shadow self. The parts of us that we don't want to accept or hide. It's not necessarily parts of us that we hate or dislike. But parts we don't want to see or show. Nor is it our bad side either. More so the hidden side of us that may tend to show itself here and there. Fenrir can help teach us to learn and accept that side of us. If properly done, it can help with personal harmony and balance. This is the point where i think both lessons of Loki and Fenrir coincide, but still remain separate.
What a great conversation that got started.... I actually didn't know Loki was I the comics and haven't seen the movies... I do think He gets a kick out of the media hype on Him.. It's a screaming shrine across the world in His name..The tweens will grow out of their phases but hopefully not before some get interested and read some real stuff...