They're always bragging about how strong, tough, macho, or Alpha they are: That's at least a yellow flag to me.

They're always bragging about how strong, tough, macho, or Alpha they are, but in a situation where they actually have to be assertive and stand up for themselves, they fall apart like a house of cards: that's a red flag.

Because they're going to take it out on you later. They're too chicken to stand up to other people, but they know you'll let them get away with it, so they'll unleash it on you to the full extent.