Can a Christian commit murder?

Knowing that murder is a crime, as well as a deadly sin, you should be making plans to exit that toxic relationship ASAP, regardless of the cost. You should also seek professional counseling. You're welcome.

Is it possible a Christian can commit murder?  Yes, if they are led by the devil.  There are surely some Christians in prison for having done so.   But it is not a Christian thing to do.  

In the defense of life or property only. My advice is to dump this guy, get a job and an apartment. If college kids can do it, you can. THEN return to school, graduate and get a better job. I recommend banking or health care.

One of the few things God's law and civil law agree on. NO.

while this person may be difficult to deal with, it doesn't sound like they're actually evil or anything. murder is a sin and it's mentioned many times in the bible, the most relevant for your situation being romans 12:19

i'm not the type of person to believe in thought crime or thought sin, so i don't think it's bad on its own to have murder fantasies, but you cannot act on them. some people are annoying and upsetting, but you've got to turn the other cheek

You're stuck living with a toxic narcissist for financial reasons? OMG, you're Melania Trump! Hi Melania! I've always felt sorry for you, and I know you'd leave in a heartbeat if you could afford to. You have my sympathies, hon.

But seriously, if you're feeling murderous and financially trapped, speak with a counselor or therapist and a divorce attorney. In that order.