This isn’t a particularly “fair and balanced” assessment. In terms of raw numbers, a) positive ratings outweighed negative ratings, so her overall rating was 3.6, which merits a “happy face” rather than frown or scowl on RMP’s ratings; b) compared to her peers in biology department, her average rating was better than the majority of her peers.
She also appeared to stimulate more “passion” than most other Biology professors at her university (only 1 other professor got more ratings than she did, while her 37 ratings typically were several multiples of the single-digit count of ratings for other professors). That per se, doesn’t make her good or bad as a professor, but she appeared to have had more of an impact on her students. In the context of having elicited so many ratings, it isn’t that surprising that at least some would have been strongly negative against her.
None of this defends her shooting anyone, but we should avoid the temptation to cherry-pick the evidence about her with the benefit of 20:20 hindsight. Pretend she’d been one of the professors shot. Would we look at her evaluations and conclude “no big deal: she was a lousy professor anyway, so her death really doesn’t represent much of a loss to the university or its students”?
That's almost certainly just a function of how long she'd been teaching there and the sizes of the classes she taught. Many of the professors/instructors included in a department on RateMyProfessor are adjuncts who only taught one course (or even just a lab or recitation section of a course) for one semester. Others are professors who only teach upper level classes with small classes of majors-only.
On the other hand, there are professors who've been teaching classes of several hundred students, semester after semester, for several years. It appears that she mainly taught a two semester Anatomy & Physiology course. Those tend to be huge, because they're a prerequisite for all nursing school programs, most physician assistant programs, and quite a few other health care-related programs.