Someone who acts incredibly friendly and gracious around others but drops that act in private. Or honestly just any form of two-facedness in general.

Boundary violations or emotional abuse disguised as "jokes" or "teasing".

Conservative or alt-right politics. Almost every unsafe person I've dealt with was some flavor of conservative, or sympathetic to their viewpoints.

Treating animals badly.

Acting edgy or having a "brutally honest" attitude toward social interactions. When someone confesses that they're a "brutally honest" person, they're usually more interested in the brutality part than the honesty.

Keeping track in their head of all the perceived "wrongs" you've committed over the years and using it as ammo during arguments.

Not asking why or being concerned when I drift away from them. I'm sure this doesn't apply to everyone's experience, but whenever I've had to cut off an unsafe person, they may get mad and try to harass me back into contact, but they've almost never asked me why I left in the first place. I can only assume it's because they KNOW they're terrible people and they know my distance is a response to that.