Only two pieces of advice:
- This is Serious Stuff. Don’t try to deal with this yourself–all three of you need some kind of professional counseling help to manage this. This is not something you can sluff off by saying, “Oh, it’s just a phase, he’ll grow out of it…” Taking pills, repeatedly, so the Evil Stepfather will go to jail, and talking about killing the Evil Stepfather with a knife, is something out of the movies–he’s living out a fantasy, and IMO this isn’t something you can just “talk out”.
Because–I don’t wanna scare you but–
Not only is your son in danger, and you are in danger, and your wife is in danger, and his sister is in danger, but also there’s a whole wide world of people out there in danger. How many headlines have we seen about a teenager who expresses his anger by stealing a car and racking it up, or putting pipe bombs in mailboxes, or simply running amuck with a gun?
Some teenagers don’t always have a firm grasp of the idea that actions have consequences–I have never felt sorrier for a kid than I did for that mailbox bomber last spring, the photos of him after he’d been in jail for about a week. When he was planning that, and he’d played that little fantasy out in his mind, it hadn’t included what it was really like to be wearing that orange coverall and using a toilet with no seat in front of everybody.
So I doubt whether Welby Jr. really grasps what he’s talking about, and things can escalate really bad and really fast, before he knows what’s happening, and people can get hurt.
So, turn this over to some kind of professional counselor or therapist, not only to hopefully lessen the tension and the danger, but also–this will sound incredibly cynical and negative but–
To make sure that your ass is covered if he does go postal and kills somebody. If you have to explain to the Authorities that you knew he was violent but you “thought he’d grow out of it” and didn’t get him some help, then God help you. 
And.
- What the Salamander said.
Repeat as needed.
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
it’s not about you and your parenting
K? 
Mother of 3 teens here who will be praying for all of ya, if that’s okay. 