You're not automatically a bad person for laughing at a rape joke. You're not automatically a bad person for laughing at
anything, because laughing is an involuntary response, and if an involuntary response can make you a bad person, the term "bad person" has lost all real meaning.
I laugh at a lot of jokes that I find
horrible. I even sometimes laugh at jokes that I genuinely don't find funny, because of the circumstances or somebody else's reaction or sheer bafflement at how bad the joke is. You can't help laughing. You can laugh and also say "No, ew, that's not okay."
For that matter, I also think there is such a thing as a joke that is both legitimately funny and problematic. I've heard genuinely funny rape jokes that were also awful victim-blaming - being funny is about buildup and subverting expectations, and it's entirely possible to do that while simultaneously being
horrible. (A lot of rape jokes are also horrible while being completely unfunny, mind - in particular, rape jokes that rely on the mere idea of rape as somehow being hilarious to the listener.)
And I also think there is such a thing as a good joke involving rape that is critical of rape culture rather than perpetuating it (though obviously you shouldn't unexpectedly spring any sort of rape joke on a rape victim when it could trigger them), like
Wanda Sykes' detachable vagina bit. The whole thing is a critique of how women live with a constant awareness of the possibility of being raped, and it treats it flippantly but in a way that's cathartic rather than dismissive.
Lastly, I think the genuinely funny kind of rape joke, even the legitimately horrible ones, can be enjoyed responsibly by people who share a mutual awareness about rape culture and why they're horrible. Rape jokes are more problematic than your average black comedy because there are so many people for whom they reinforce genuinely held toxic ideas, but within a closed group where the awfulness of rape is as obvious to everyone as the awfulness of murder, they become more or less equivalent to murder jokes (insert your preferred subject of black comedy here). Shadey and I were just playing Cards Against Humanity with my brother and his wife; we're all feminists, and we all came up with the most horrible shit and laughed ourselves silly at it, and I assure you no harm came to the cause of feminism in the process. Laughing at awful things in a mutual understanding of exactly why they're awful is cathartic and wonderful and basically human nature - it's a way of coping with the awfulness. Deciding Nobody Can Laugh at This Under Any Circumstances Ever is unproductive and unnecessary - focus on the ways it does actual harm.