You said this in a much nicer way than I would have. Kudos to you.I don't think life experience, interests in "creepy" things, personality disorders, bad childhoods, or sadistic/masochistic tendencies have anything to do with personality type.
Some ISFPs seem innocent, but others are sitting in their parents' basement with their girlfriend cutting themselves. I mean, Salvidor Dali was an ISFP, don't you consider him a bit twisted?
If there's anyone I stereotype as being so sweet that butter couldn't melt in their mouth I actually think of INFx first, not ISFPs. ISFPs I think of as being more realistic. But of course they aren't always...they have tertiary Ni.
Sensors don't lack an inner world or the ability to like strange or dark things.
I had these kinds of thoughts when I first started looking at personality theory, except I specifically had them about SJs. I thought all the boring, bland people were SJs and that no one interesting or artistic or cool could be SJ...so isn't it funny that as it turns out, my ESFJ ex is actually uber-SJ, I can't see him as anything but ESFJ in function theory, plus he tests as ESFJ in the four dichotomies. But at first I thought he must be ESFP because of his obsession with horror movies and punk music.
Similar for my ISFJ friend. I thought he was ISFP at first, and he had to convince me otherwise.
Stereotypes don't really work for personality types, it's a terrible way to type people, and I guess we all have to learn it.