In fact, before I first heard the Grateful Dead, I too had concocted an image in my head of them as a group of evil, devil-worshiping, psychotic, violent criminals; I could only imagine what kinds of terrible music they'd have to their name.
The first time I heard a Grateful Dead song, however (it was "Friend of the Devil", still one of my favorite songs to this day), I immediately knew my preconceptions about the band were entirely wrong. The song was recorded with all acoustic instruments and it made me feel like I had gone back in time about a hundred and fifty years, not because it sounded old but because it sounded pure. I soon confirmed that in fact the Grateful Dead was not a death metal band, that they didn't (as far as I know) worship the devil, and that they didn't wear make-up or kill animals on stage. Quite the opposite, in fact -- the band seems to have been a strong proponent of love and peace and purity. And most importantly, they crafted what I consider to be great music that took elements of older music styles (particularly blues, bluegrass, and folk) and molded it into something unique and new. So, throw out any negative ideas you may have about what the band stands for, and give their music a chance. I think you'll find it's quite enjoyable.
The reason I've featured the Grateful Dead in this post is that I'm going to see the The Dead (which features the surviving members of the Grateful Dead plus Warren Haynes on guitar and Jeff Chimenti on keyboard) in concert tomorrow night. I've never had the chance to see them, so I'm excited. I've also created a lala playlist of some of their more user-friendly songs. If you like the playlist, check out the album American Beauty in its entirety.