honestly this whole video is really interesting because every time you "fail" mettaton's set ups, you get dialogue of alphys basically beating herself up and freaking out about being a bad person and feeling guilty, but once you get to the MTT news segment, and after you've seen that every time Alphys has messed up, Mettaton has been able to sweep in and help her, her switch up to suddenly saying "it's mettaton's fault if you're in danger, not mine" is so interesting.
one thing is that she's so invested in trying to believe that she's not a bad person when she feels horribly guilty about what she's done in general, and including to you and to mettaton. her entire plan hinges on making her feel good, she's got her best friend piling compliments on and she's fishing more compliments out of you. i think when people feel guilty they tend to try to offload that guilt wherever they can, especially onto others, because feeling guilty SUCKS. and... i think on a deeper level, this line is kind of representative of the way alphys managed to convince herself that mettaton was the problem in their friendship falling apart, specifically. the lab entry about how he won't talk to her anymore with the reason being that he "made it big" shows that she kind of needs things to be his fault, he did something wrong, he's negligent, and I think she feels like if they were still as close as they used to be, she would have someone to lean on and support her with the amalgamates. i think she feels a bit resentful that he isn't helping her with her mess, which is a very real way to feel when you've got something going on that is so terribly out of control. you just want someone to make it better. obviously, mettaton has more understandable reasons to be growing distant from alphys beyond just "making it big."
it's also that mettaton is clearly her yes-man, she's used to him saying yes to her and making her feel better. and this is supported by him even agreeing to this, and by his own admission even when things are better between them, he doesn't tell her "no." so in a way,
he kind of exists as a bit of a shield for her. when you mess up the can of artificial soul jetpack minigame, she can tell that he's coming in with the right line at the right time to save her and to keep her bit going and shut down her negative feelings. (it's also how mettaton treats his own negative feelings, by avoiding them entirely — literally "pretending the mystery key isn't there" when it's right in front of him in the ex fight). he's a good friend, but i think his solution with problems is to try and pretend they're not there, and that especially includes when alphys is involved. so, if there's no mettaton buffer for the problem, then it's his fault anyways.
it's also apparent how unhealthy her mindset is that her confidence is entirely reliant on everything going right, mostly because she's horrified that up until now everything in her life has gone wrong and she is traumatized. she literally can't handle another thing going wrong, another thing being her fault, because at this point it's a life-or-death matter to her. she is actively doing all of this to try and walk herself back down from a ledge, and it works too well, but that doesn't mean it's healthy. building her confidence in this way harms her in the long run and harms her relationship with her best friend. eventually after true lab she's able to notice that, but i think the way that the hotland segments involve her throwing mettaton under the bus to basically pull herself out of the hole she's in, thereby actually worsening said hole, is sooo interesting... my shaylas need therapy....