Ngl, he is a douchebag.
But so was Jim Morrison, and John Lennon and Elvis and pretty much every damn musician ever. Finding musicians, especially in rock and its offspring genres, that aren't shady is harder than it should be in an ideal world. Even John Denver and Pat Boone had their moments of dickishness. And you don't even want to know what contemporary christian musicians do when they're out of the public eye, so even people that claim, in their music to be all about being decent and loving (their) god can't be assumed to be worth a damn.
For me, with any genre, the question isn't about the musician, it's about the music. Manson, while he covered some dark topics, didn't write songs encouraging raping people. Not that such a song wouldn't have been able to be a good song, but I wouldn't have listened to it. John Lennon didn't write songs encouraging people to be abusive to their spouse or abandon their kids.
Hell, even Phil Anselmo hasn't put out songs espousing his batshit version of white supremacy, and he's the kind of guy you'd expect to do that.
That's the line for me. IDGAF about the artist, so long as the music isn't centered around their douchebaggery. I might never buy an artist's music because they're a douche (Manson included), but I might pirate it.