I'm confused about their use of the word "beta". Are they using it positively or....?
It gets convoluted, but from what I can tell there are two types of men who hate women: alphas and betas.
Alphas consider themselves superior to women and able to easily manipulate them. They consider men who respect women to be beta males...weak, ineffectual, etc.
Guys who are socially awkward and hate women for, basically, not giving them pussy call themselves betas. Even though it's a pejorative, they wear it proudly, like geek.
In that thread, there seem to be both alphas and betas. The alphas are using 'beta' with a negative connotation, like the guy who tells him that some beta males may try to protect the women. The betas seem to be using 'beta' in the context of solidarity with the soon-to-be shooter.
If you notice, one of the posters who encourages him to be 'legendary' has an avatar with the photo of the guy who shot up those kids at UC-Irvine because he felt that women weren't giving him a chance.
Guys who identify as betas half-joke about the 'beta uprising' in which they kill guys who are not socially awkward (alphas) and women who won't fuck them.
The whole thing is just skeevy as hell.
ETA: As Guy alludes to, the alpha/beta bullshit emerged from the pick-up artist community and is now widely used by men's rights douches as well.