“Evil” is different in pagan traditions than it is in Judeo-Christian traditions. Evil is what one does, not what one is.
I work with trickster gods and spirits. Fenrir and Satan are both a part of that fraternity, in some way. The question is: do you worship them blindly and let them lead you down a dark path, or do you work with their energy to find the dark in the light and the light in the dark? Tricksters are…tricky. It takes a bold heart to work with them and NOT walk a left-hand path.
As for “backfiring”, yeah. These guys function largely as challengers to black and white thinking. If one works with them and cannot release themselves from black and white thinking, it is very easy to get lost in the darkness.
I would ask why you find connections with these and not other deities. What is going on with you already that you need to explore and work through?
Link to the original question on Quora.