Spoilers below.
I don't know anything about this pregnant girl except she was in a car accident, her husband died, and you guessed it: she's pregnant. She is not portrayed as a full-fledged human being. There are no chances for levity or humor in this film. She can't smile or make a joke, not that one normally would after such tragic events, but even in the worst of times, we often find something surprising that causes us to show the stark opposite emotion of what you'd expect. I'm sure she would reflect back on some positive memories with her lover. You need something to flesh her out so we can get to know her as a person. As someone we understand to be real and not just a puppet on the stage. We really don't get anything about her, and that one dimensionality brings us to the next point.
Inside basically has one mood throughout: depressing. Everything is grim and dark. Her house looks like a living nightmare simply because of the way it's lit, which I like since after losing your spouse and returning to where you two used to be happy together, it would become an awful terrifying reminder of what you've lost, but to convey such material, you need contrast. You'd have to show the house before and after so we see and feel the change (think of when RoboCop goes back to his empty home). One mood can work, but it's tricky. John Carpenter's The Thing pretty much has a bleak mood all the way through; however, you care about the characters, and some of their antics, like MacReady pouring his drink in the computer chess game after it "cheats," are pretty funny so it's not all one note. Inside, on the other hand, is always the same, and unfortunately, it's only concerned with surface details.
Let's look at the antagonist. You don't learn anything about her until the third act of the movie. Up until then, all you know is she has baby fever and she likes scissors. So yeah, she's batshit insane. She's also not too bright since she quickly alerts the pregnant girl to her presence by standing right outside the house in plain sight to where the girl calls the cops. Good move. The audience is left to wonder who the hell this woman is for 80% of the film. So she's basically the shark in Jaws up until the end, which could work if I knew the protagonist (Brody? Quint? Hooper?) but I don't. I don't know anyone so I'm not invested in what's happening.
Another monumental flaw is the way the movie strains believability with genre cliches. The pregnant girl kills her own mother by mistake. Sound familiar? It should. The same kind of thing happens in The Strangers, The Descent, and about a billion other movies. It's extremely hard to sell that, I've seen it before (especially recently), and even if it manages to work (I don't think it ever does although it didn't bother me as much in The Descent), it's still annoying. If you want to read some deep message into how the girl is taking the place of her mother by killing her or whatever, quit kidding yourself. That is far too obvious to be interesting, and if they intended that to be a theme, it's not developed. The mother-daughter relationship is not explored at all in the film. As soon as you meet the mom (and by meet, I mean see, because you don't get to know any of these characters), she is killed in one of the most contrived deaths I've ever seen. Of course, the girl would not see her mom before opening the door and stabbing her.
The other ridiculous cliche would have to be the stupid cops. Do they call for backup even after they realize something is horribly wrong? No. Do they have radios or cell phones on them? No. Do they behave anything like real police officers with training? No. Do they all get killed by being complete idiots? Of course, they do. What's sad is they fool you into thinking they might be doing something smart with the cops, because they have one realize that the lady answering the door is not the woman who called for help, but then instead of immediately alerting his co-workers or calling for backup, he turns into a total moron so he can die, and the other officers follow suit. You'd think one might possess some intelligence but no.
You also get the villain that never dies. Evil witch basically gets her whole face burned off (the makeup looks outstanding, but that's the only positive) and yet she still survives with enough strength to overpower the protagonist and kill her. Yes, it's a happy ending (actually, I like the last shot, but I hate the movie). Now, let's go back to the discussion of bad taste and grossing out your viewer.
Have you ever wanted to see a makeshift C-section with scissors going in a woman's belly button? Well, you get one in all its disgusting glory. Does it really add anything by showing it? No. Is it cheap and exploitative? Yes. Is it completely predictable? Yes. Do they give you a nice insulting overview of the aftermath with our dead protagonist? You bet they do. Somebody must really hate pregnant women, because they have no sympathy at all, and they don't have any for the audience either. Got to be extreme, right? And what's more extreme than cutting out a baby with scissors? This is not what horror is supposed to be, and it's just giving ammo to the genre's critics. I could not defend this movie to non-horror fans, and I would not try. It's a sad excuse for storytelling that fails on every level. There's nothing smart about it.
So by the end, we find out the antagonist was in the same car accident, and she was actually pregnant too, but she lost the baby because of the crash. This would be a good twist if the protagonist had been developed, because they basically switch roles. We might have rooted for the antagonist had we known all along and had she been built up as a sympathetic character although she's probably still too psychotic to win us over. But it turns out this twist is mind-numbingly stupid. The protagonist tells us that no one else survived according to the authorities. So they lied to her? Everybody who arrived at the scene? All the doctors and nurses at the hospital? Why would they do that? That doesn't make any sense. Of course, someone would've told her. So they didn't know someone survived? How is that possible? There would be the other vehicle with evidence that someone left the scene. I'm sure she would've been injured (she did lose the baby in the accident) so blood would've been left behind. If she was the driver, obviously, they would've known there was a survivor. If she wasn't, she still had to get out of the vehicle (which I imagine she couldn't do by herself after such a bad accident), and if she was with a spouse, they would've known to look for her. Even if she was just dating the guy or he was a friend, their families would've known when they were notified. It just doesn't make any sense. Did the protagonist's family not let the authorities tell their daughter the truth or did her parents lie to her so she didn't suffer more trauma? That is still such a stretch, and it doesn't make sense either, because the authorities would've had to tell the parents so they would've recognized the woman in the house. Why is this line even in the movie? It just makes the whole thing unravel.
One last thing I want to talk about is the CGI. Most of the film uses practical effects, which are quite good, but then you get these awful jarring shots of a CG baby in the protagonist's womb reacting to the mother getting hit, kicked, etc. You shouldn't do these shots if you can't make them believable. They're like a bad cartoon. They don't fit at all. They don't add anything anyway (I didn't expect the baby to be smiling in there), and since they look so fake, they instantly take you right out of the movie.
Every good film has characters you care about. Characters that are actually developed. Making a film to be extreme is just silly and pointless. A good filmmaker understands the difference between good taste and bad taste. They're saying something or expressing something meaningful to them, and it should be communicated well enough to where it becomes meaningful to the audience as well. The whole movie shouldn't fall apart if you spend more than two seconds thinking about it. Anyone can disgust somebody. Just chew with your mouth open. That's the cinematic equivalent of Inside.
