Is it a red flag when a man hates his mother?

Depends on what she has done to him as even blood and flesh is capable of doing nasty things; if the reason is petty, however, then yes, it would be a red flag. Keep an eye out for people who don't like their family and those who are disliked by their family--something usually isn't right.

Hate is a very strong word, especially if he was only offended. I think it depends on what she did. Just be cautious.

Uh....offended????