As said in Simon's answer, Glassdoor reviews should be taken with a grain of salt: Those upset with the company are more likely to write about their experiences than those who are satisfied. Therefore, the fact that "most" of a very small number (25 is "very small") of reviews are negative should not in itself be a huge red flag to you.
That said, it is not likely that people will randomly sign up for Glassdoor to write negative reviews about a company in which they have no interest. It is likely, at the very least, that the content of the reviews that do exist are true; it is simply the scale that should be taken with a grain of salt. For me, what I would take away from that is that there are bad teams that exist within the company, and to decide for yourself what the likelihood is of you being placed on such a team (e.g. does your future job description match the job description of the person who wrote the review? If so, you might be on that team) and how you would feel if you were.
As for what you should do about it, it depends on what stage of the interview you are in. Personally, when I interview, one of the first things I do, even before the phone screen, is to look up the company on Glassdoor to see what people have written about. Then, if I see anything negative there, that becomes part of my screening questions for the company, e.g. "I looked you guys up on Glassdoor, and I saw a large number of reviews that said xxx about you; can you speak about that?" or something like that. If the reviews are particularly pointed towards a certain role/direction/group/person, then perhaps asking them to a particular person would be better (e.g. if you get an interview with one of your future coworkers, you should ask them if this is something they have seen/heard, or even if they have noticed low morale in their team, or something like that). In addition to addressing the issue head-on, it shows a few things:
1) It shows you have done your research on the company before attending the interview. No interviewer wants to hire a person who just grinds through companies without looking at information about them.
2) It shows them that you are actively looking for a position which is a good fit for you, not just "any job".
3) It shows them that these negative reviews are having an impact on their reputation, and that they should address these issues if they want to hire good talent.
That said, if you are in the later stages of the interview process, there is increasingly less you an do about it. As an extreme example, if you have the offer in hand already, you have fewer opportunities and points of contact you can ask about it; you're basically limited to only discussing this issue with the HR rep who you are assigned to, and as the saying goes, "HR is not your friend"; HR will say whatever they need to say to address your concern and get you to sign the contract, even if that means stone-faced lying to you. The more you can ask to the rank-and-file and not ask to HR or middle management, the more honest the answers will be.