So if you are up front from the beginning, with your words, that's one thing.
What you do with your behavior might send mixed signals. I'm having a hard time believing these hurt feelings are just coming from a pick up play or one night stand situation as you describe.
You say: "Predatory? I expect them to open up and be very vulnerable in front of me for kink/sex but afterwards carry on without an attachment. Is this cruel or naive of me?"
As long as you are clear about this, it's not cruel or predatory. But, if you cultivate a lot of emotional intimacy so that you can get what you want out of the interaction (tell them you love them so they'll let you slap them in the face 80 times and put your knife in their mouth), then yeah, that's a little exploitative.
And no, you're not responsible for their feelings. Let's say you do cultivate a lot of emotional intimacy and they do fall for you, and you don't want them to fall for you, you just wanted your knife in their mouth once. You still got what you wanted, and if you're not their friend and you don't have to see them ever again, it's whatever. Why do you care how they feel at all?
If you have to see them around in the community and at parties, it can get a bit awkward. Which is unpleasant. But again, in my experience in kink/leather communities, the person with the hurt feelings is the one in the wrong, so they'll probably keep quiet and self-select out of the community, so I don't see how this affects you greatly.
In my experience, "we're going to do this until I get bored with you" means an affair over the course of months, though, not something that is descriptive of a one-time thing. And if you're having an affair over the course of months having really emotionally intimate vulnerable sex and play, then of course someone is going to get attached.
And I mean, you can literally say "I'm using you for sex and attention until I'm bored and/or someone better comes along," but then, most people probably won't agree to that directly because it would push on their dignity and pride. Idk...