I haven’t read the manga, but based on the show, the end is…alright.
The Trigun Maximum manga wasn’t finished at the time the series aired, so their endings are unrelated. The last 4-5 episodes of the anime are complete originals. (There were two different manga series, and I think the original Trigun manga had more of the goofy-villain-of-the-week stuff, while Maximum, which picked up where the first series left off, was more serious.)
“No evidence of foul play” just means there’s no proof that anyone set out to kill her on purpose. It doesn’t mean that there’s no evidence of negligence, although the police may not think they have enough to bring criminal charges in that vein, either. The burden of proof in a criminal court is high, and there’s no point in wasting the court’s time if there isn’t enough evidence to convict anyone.
Civil court has a much lower burden of proof, and I hope the victim’s family sues Walmart’s pants off, because it really does look like there was an issue either with the equipment or with employee training that contributed to this.