A meta example: how do you take a classic "monster" movie like The Invisible Man and make it an original take on the source material? The concept that Blumhouse goes with for this movie ended up being both brilliant and horrifying at the same time. Not only does it update the science-fiction elements of the story, but it also uses the source material as a way of telling a more modern tale that is relatable to a lot of people.
How did Cecilia know for a certainty that Adrian really WAS the one behind it all, and not Tom as he claimed? There are two clear signs: after Cecilia stabbed him with the pen, Adrian's suit continuously glitches during the hospital scene, while Tom is wearing a suit that isn't glitching (and, just as importantly, doesn't appear to be even slightly wet, despite Adrian being drenched at the hospital); and Tom manages to somehow beat Cecilia back to James' house, not just by a little bit, but by a wide enough margin that he's able to attack Sydney, and then nearly beat James (who had to get from his precinct to his house after receiving Cecilia's warning call) to death, long before Cecilia arrived to save them both. This second point would only work if Tom was actually already at James' house beforehand, likely waiting for the go-ahead from his brother to kill Sydney. Likewise, the extra time would have allowed Adrian to return to his house, stow the suit, seal himself in the basement, and tie himself up before the police arrived.
Another thing that's rather suspicious, is after Tom is killed, a police team enter Adrian's house, and Adrian is heard calling for help through the walls. Strange how nothing of the sort is heard earlier in the film, when Cecilia was snooping through.
We never really hear about the specifics of Adrians abuse, since Cecilia leaves him at the beginning of the movie. Throughout the movie, however, Adrian uses a ton of abuse tactics that he likely used when they were together to drive her crazy, and are common tactics with abusers in general;
Hiding her important work for a job interview may seem quite petty, but it serves to make her seem forgetful or ditzy to the interviewer, and make it hard for her to earn her own income. After all, if she got a job, she wouldnt be dependent on Adrian anymore.
Before doing anything excessively violent, Adrian does small things to make Cecilia look crazy or mentally unwell to James and Sydney, such as turning up the heat on the stove,or drugging her to make her seem impaired. Abusers tend to make their victims look crazy so that its more difficult to believe when the seemingly well adjusted abuser is accused of harming them.
Sydney thinks Cecilia hit her in the face, which is the breaking point for James as he tells Cecilia to leave. Its not uncommon for abusers to act as if they were the ones being abused, to make the victim seem insane. Of course, Adrian cant make himself seem like a victim, so he set up a scenario where it happened to someone else.
When Tom tells Cecilia that Adrian needs her because she doesnt need him, it sounds like hes just saying how much of a control freak his brother is. But its more poignant than that; Adrian never loved Cecilia, and the only reason he does what he does is because he wants her purely out of spite. Adrian is a rich, handsome, and physically fit man, yet his horrible personality and sociopathy make him an emotional drain on both his brother and his ex-girlfriend. In other words, Adrian genuinely seems most happy when making others miserable. Meanwhile, Cecilia, at least at first, has her sister, James and Sydney as a support net, and has the wherewithal to get a job and start a career (before Adrian ruined it, at least), and despite her trauma, moves on from Adrian quite quickly after hes dead. For all his power, Cecilia doesnt need him, but he needs her so he has someone to abuse.
It could also be a Take That! to real life abusers, saying that abusive people are a drain on those they harm.
Tom is a foil to Cecilia; whereas Tom willingly obeys Adrian and acts as his pawn in hurting others, Cecilia refuses to submit to him and ends up his target. Fittingly, Toms story ends when hes killed as a result of following his brothers orders, whereas Cecilia is the one that kills Adrian for all he did.
While Cecilia killing Adrian at the end, and gaining her freedom in the process, is extremely cathartic, her manipulation of James in constructing her alibi has likely irreparably damaged their friendship, with his realization that the trauma Cecilia suffered drove her to behave as sociopathically as Adrian himself.
Given that Tom withdrew the Trust from Cecilia after she was committed, how likely is it that she's still cut off, even after Adrian was "found" alive? Tom mentions that he doesn't look forward to telling Sydney that the account that Cecilia set up for Sydney's college fund was now gone, and we receive no indication that the Trust was reinstated after Tom's death. Once news of Tom's actions broke, the will would likely have been invalidated, as, unless Adrian said otherwise, Tom's partners may have decided that it was created under duress (as per Adrian's story). Even worse? Adrian almost certainly didn't expect that Cecilia would try to kill him, so it's possible that there is no alternate will yet, or if there is, it probably wouldn't have been under the same terms as previously.
Though to be fair, Cecilia may not want the money anymore. It would certainly help with childcare, but considering it's her tormentor's money...
Usually people justify taking blood money by using it to benefit others like themselves.