how do you decide whether a character green flag or red flag?

I personally always see it as the way they treat their partner. so even if he’s unhinged and kills people, I would still see him as green flag if he treasures the mc and never manipulated / abused / raped / SA / or hurts the mc physically and emotionally. but I often see people classify them by their personality itself. for example if the ml kills people for the mc they would see it as at least yellow flag. what do you guys think?