I actually can't think of any group where that would be an accurate description

No real group, or even an imaginary group? I mean, take the "misanthropic losers" you described (and for the sake of debate, let's assume your description of them is completely accurate), and imagine exactly the same group with genders reversed. Would it be okay to call those women publicly "losers"?

Or perhaps "loser" is a gendered slur. (Something like the word "slut" that you can use to offend women, but if you try it to describe a sexually adventurous man, it somehow does not have the same shaming power.) In which case, saying that the "Red Pill" readers are losers contains almost as much information as saying that they are men.

Sure, lots of folks blame the failings of society for their comparative lack of success, and that's sometimes unhelpful. But even that is a lot better than just complaining about how all other people - most specifically including women as well as 'alpha male' other guys - are somehow evil and stupid.

Complaining achieves nothing, and people who complain without achieving anything are, yeah, losers.

How about a group that achieves real results? For example, there is a controversial movement, in an obscure part of the "manosphere", behind a blogger Valentine Solarius, often criticized by feminists for writing things like "to be female is to be deficient, cognitively limited"; "the female is completely egocentric, trapped inside herself, incapable of empathizing or identifying with others, or love, friendship"; "her intelligence is a mere tool in the services of her drives and needs"; "the female has one glaring area of superiority over the male - public relations; she has done a brilliant job of convincing millions of men that women are men and men are women"; "every woman, deep down, knows she's a worthless piece of shit". -- He writes a lot about his desire to kill women. Actually, he attacked and almost killed one woman for not responding to his e-mail, but she survived so he only spent three years in prison. He seems to be a popular person among some men politically influential in the Republican party... so, let's assume that his friends really succeed to create a political movement, change the way society perceives women, change the laws as they want to have them, etc. Then, they would no longer be losers, would they? Now, would that be better than merely blogging about the "Red Pill"? (See his blog for some more crazy ideas.)