Ofc, but I don’t even see a point of saying it in your head. Like sure it’s your own thoughts, but what’s the point? Just to have the “pleasure” of saying it?
And then those same people who think that’s okay would be saying “Oh, this is so horrible. What this person is doing is not okay” when they actually go out murdering people. Like didn’t you just say it was okay for them to plan it in their head where no one can hear them? Bottom line is, even thinking about legitimately murdering someone (even if it’s hypothetical), in my opinion, is not okay.