There are some people who immediately give off an inexplicably bad vibe, and while first impressions can be deceiving, there's something to be said about the accuracy of a strong gut feeling.
Oftentimes, when our internal alarm system tells us someone is a walking red flag, we don't get confirmation of how right we are until later. It can feel judgemental or irrational to follow a whim when someone hasn't yet shown their true colors, but in situations where you're correct, it's far better to be safe than sorry.
When I was 14 I was walking home from the bus stop. There was this guy who drove by, saw me, and immediately pulled over and tried talking to me. He must have been at least in his late 20’s, normal clean cut looking guy, didn’t strike me as someone dangerous but I got a shiver down my spine.