This week's assessment comes from Elizabeth, who is writing a YA mystery/thriller set in present time. Homeschooled antihero Donnie* comes from a high society family. His dad was rarely home, fairly negligent, and verbally abusive at times. Donnie grew very attached to his mother until, at the age of 16, her Parkinson's disease changed her, making her bed-ridden and ill-tempered. He encouraged his younger brothers to never leave her side. Vivian is hired as a maid to take care of things around the house. Donnie becomes quickly attached to her, because she reminds him of his mother pre-Parkinsons's. When this attachment morphs into romantic feelings on Donnie's part that aren't returned, Vivian quits her job as their maid.
* Names have been changed to protect the fictional.
Elizabeth wants to know: Would it be realistic for Donnie to completely lose it when Vivian quits and convince his brothers to help him kill her?
What you're describing here for Donnie does seem to be a fairly large leap, given what little info I have. The antisocial urge to kill someone doesn't arrive in a vacuum, and there should be a few other characteristics that Donnie should have so that this wouldn't surprise the reader.
I say surprise because that's exactly what it did to me when I read it. I can imagine a 16-year-old boy who imagines himself in love with the maid to feel a lot of emotion when she "spurns" him (quits the job). Likely he'll be overwhelmingly embarrassed, but people tend to not exhibit embarrassment as a general rule. Instead, they run straight to anger, which is the secondary emotion, but easier to handle. So yes, I'd imagine him getting fired up about her, maybe even wanting to prank call her or egg her house or some other juvenile stunt.
But kill her?
Since Donnie is 16, he's too young to have Antisocial Personality Disorder (APD). But if he had some symptoms of Conduct Disorder (which is usually a precursor to APD), then his homicidal urge would make more sense, especially if you have him influencing his brothers and staging an all-out premeditated attack against Vivian.
But not to make him this weirdo, either, right? So I looked over the breakdown of Conduct Disorder and began imagining this boy Donnie. The teen angst due to Vivian's betrayal....I think you're going to have to give the reader some clues up front that not all's well in Donnie's mind.
For example, one criteria of Conduct Disorder (and he'd only have to meet three to get the diagnosis) is deceitfulness of theft. If Donnie snooped around Vivian's car or even her personal home or car and tried to take something of hers, something that he'd value, then this would tip the reader off that he's got a dark side. (Now that I've typed that...that's totally what you'd have to do. You might definitely have thought of some ways to alert the reader in this regard, but the sketch was absent those details.)
Let's say he took something of significance to him and of little significance to her, like her chapstick or pen. This would meet the criteria of stealing items of nontrivial value without confronting the victim. it could also be a really good way for Vivian to start getting the creeps...her stuff is missing, but it's little stuff so she can't very well make a big deal to her employer (the dad). She could have just misplaced her pen or dropped the chapstick. See?
The bit about him being this charismatic leading brother could be worked in to fit the diagnosis, too. Many charismatic people lie on a regular basis. I mean, we've got people claiming to be God off and on, and many religious group members follow the lies to their death. This is charisma. Deceit usually doesn't take the unappealing roadway. So if Donnie could sort of be like a little conman with how he treats his brothers and others, lying to them to obtain favors he wants (which could still be twisted and have him get these favors not so much for himself, but for his ailing mother), then this would be another characteristic.
Once Vivian leaves, he might should try to destroy some property of hers first, without jumping straight to an assassination plot. If you show him ramping up, then the reader will ramp up along with you. You won't have to spell it out, they'll get it and say to themselves, "This guy Donnie is off. Not at all how he originally came across." (And so many psychopaths are the same way! You'd never know them if you passed them on the street.)
The one area that I think Donnie might not meet much criteria would be the aggression to people or animals, unless you can make him more intimidating or bully-like when trying to get his brothers of even Vivian to do what he wants. But I honestly find the other, conman stuff to be a bit darker...not quite so obvious as outward aggression. This incongruity makes his choice of murder to be all the more shocking (in a good way...not surprising as in, "what on earth?) to the reader.
That's all I got this late Monday evening. Been a long weekend! But I welcome any additional questions in the comment section. I hope this helps!