Several people who have not gotten the red skull, including me, got the message [09:20] Tutorial: You have been marked with the red skull of a player killer. You may find that people are wary of you now. To get it removed, you may speak to Io Flotto in Semos temple.
This seems to happen if you were training with another character and then something else killed them later when you were on another screen, but the thing that killed them did less damage than your character did during training (or in some cases, PvP combat).
Anyway, the bug is that you get a message saying you have a red skull, but you don't actually get the red skull. I don't think people should get the red skull in this circumstance when they didn't really kill the other character and were on another screen and something else killed them. So the solution is to have this message about getting a red skull only happen when you ACTUALLY GET a red skull, NOT to have everyone who gets this message also get a red skull, because a lot of people seem to get this message about getting a red skull mistakenly, when they weren't the ones who killed the other player, just maybe hit them a bit during training or combat or something and then were off on another screen when the other player was killed by something else (in my case, about 5 minutes later).