Agreed. When I'm interviewing someone, any negative remarks about past employment (or anything really) would be a huge red flag, regardless of the actual merit of the remark. In my opinion, this is just not something smart people do (being negative during the interview), and my main objective during an interview is to determine how smart one is (not to mention being pleasant to work with). They can bitch about their past employers over beer in a bar - after they've been hired.
#1-3 are about how the interviewer will likely think if the discussed remark is said in an interview; they are not my conclusions about the OP.
It would have been more accurate if I'd stated that was a risk being taken by the OP in the interview rather than a certainty. Though I expect that would have made the point less clear to most readers.
#3 correlates with #2 (not #1), and this relation is not specific to the OP.
#2 may not be true of the OP, but an interviewer who thinks it based on things said in the interview, is likely to think #3 follows.
This is a risk the OP is advised not to take if they are trying to make a favourable impression.