Sounds like Sam has a lot of anger, and some is displacing onto you. But because he can't deal with his emotions, means you have to be even firmer about not allowing him to make you the bad guy.
I suspect he might be feeling guilt that you are elsewhere and he is in your house, plus anger over whatever happened, plus anger at how out of control his life is. Accept none of this. You have done nothing that should bring any anger down on you. Polish your spine; I suspect it will get worse before they are out, and I suspect it will damage your relationship long term*.
*Don't mean your relationship is doomed, just that he may need serious time to get over all his anger issues since they are so out of control.