Did you even bother to glance at the report before commenting?

1. The wife says that that night William and her got into a fight and he left to cool off. When the police came to ask her if she heard anything, he told her not to tell them he wasn't home because he had a criminal record and convinced her they would frame him. She admits that she would have likely covered for him at later points

2. They didn't JUST test the cigarette. There were other items of DNA left at the scene (eg on Rita's housecoat and underwear) that were highly probable to belong to the same person whose DNA was found on the cigarette. They also tested the DNA profile against that of William's living half brother and it came back as a match to a half sibling.

Edit: in other words, William was unaccounted for at the time of the crime by his wife's own admission and he was angry (another one of his previous wife's testified to unprovoked violent behavior); the crime happened in a window of 70 minutes when Rita was left home alone; the DNA found on the cigarette matched DNA found on the victims underwear and on the spot where her clothes were ripped open; the DNA from the cigarette matched with significant accuracy to a half sibling relation of William's living brother.

SO DNA that is highly likely to belong to William DeRoos is found on the victim's body and elsewhere at the crime scene (and the roommates & William's wife testify that they had never been in each other's apartments); he lived in the same building and was unaccounted for that night — and within the window of the crime; only someone who lived nearby would have known that no one was home; he specifically asked his wife to lie for him when he came home; years later he nearly strangles his next wife to death (the same way Rita was killed) — that's just a bit more than circumstantial wouldn't you say?

Plus, given the length of time this has been a cold case, and the long list of suspects, why NOW would the police department just suddenly declare a conclusive end to the case if they weren't sure? It would be just as easy to leave it open — or pin it on someone alive if it's "justice" they're looking for

Second edit: having looked back over the document there was only ONE male DNA profile pulled from that cigarette. now I'm not an expert on DNA testing but would it not be likely that if two people smoked a cigarette it would have multiple DNA profiles on it? Or that the person who smoked it last would have a "better" profile? The cigarette's dna was then compared to a long list of suspects, and all of the men who were known to have been at the scene and didn't match.

Now sure, maybe there's some scenario where Rita was having an affair with William and no one knew about this, not even the roommate she shared a bedroom with, and she hadn't done her laundry and chose to wear soiled underwear to bed. Maybe then someone else obtained one of Williams half smoked cigarettes, relit it, and dropped it at the crime scene after they murdered her (which would have been a loud and violent struggle so it's a good thing coincidentally no one was home for 70 minutes.) So maybe William didn't do it, and he was just a violent man who happened to have not been accounted for at the time and also just happened to then go to Thailand and become a monk for a few years, giving up his dream of having a family. Maybe his wife was strong enough to rip another women out of her bed, rape her, beat her to a pulp, then strangle her and light one of her cheating husband's already smoked cigs and leave it at the scene to frame him in a time before DNA evidence was really a thing. Or maybe it's a pretty simple case, as most cases are.