Not really.  Having said that, we all have murderous feelings towards someone occasionally that we don't share, but to have them continuously and to have a long term murder plan is not normal. What does the psychologist make of your SS's statements to his mother?  Has he/she discussed them with you and DH?  

Hi, 

I am not for sure how long you have been involved with step-kids and Dad, but either way, kind of, this was not handled correctly. I sort of know what you're going through, except my step-son usually goes with how he doesn't want to live anymore, but he won't do it because Mom would be really sad, hit it works with Mom every single time, hence the spoiled behavior at it's making. 
There should have been consequences for his behavior towards "everyone", but I'm assuming that it's you, and maybe dad also? How does he get along with Bio Mom and his Dad? How about your relationship with him? Has it always been a negative reaction towards each other? 
The only sentence that I agree with him saying is that it's okay that you feel that way! That's it because it is okay for kids to show their feelings and not be shut down by a parent, but absolutely not should he have said what he did and how it's okay to think you want to kill someone. There should have been a lot more discussion with the kiddo than there was because (yes, you're probably right that this is for attention) there could be that possibility of it happening if someone that he was needing the attention from would go for it. That's the scary part is not knowing what could or could not happen. I don't know how long who this happened, but try talking to him about your thoughts on what happened. If he begins to back his kid up, don't let him by saying something positive about his kiddo, such as, he's such a good kid and it concerns me on why he is speaking like this... it's a difficult situation and working as a team of parents is what will truly resolve this for the kiddo. 

Not even a little bit. This isn't an episode of Dexter. Your DH isn't going to teach SS how to only hunt murderers and somehow get him a job in a police department so he has ready access to information on who to kill. SS has problems, and telling him to HIDE those problems versus work through them (or admit to them so he isn't a threat to himself and others) isn't helpful.

Honest question: do you think your DH has similar thoughts/feelings that he "hides" to avoid being "locked up"? Do you feel safe with DH and SS? As an outsider, I'd be f**king terrified if I knew my SS wanted to kill me and my DH just said "that's okay that you want to kill us, just don't tell us about it".

Far from handing it correctly, DH supported his toxic failed family spawn's toxic crap.  DH is an idiot.