My father had PTSD (undiagnosed until very recently). My husband may have symptoms of PTSD.
More and more I tend to think that 'states of mind' are more or less 'contagious'. By this I mean that if you are with a person who is (or gets) very defensive and stressed, you too will become defensive and stressed. And your body and mind will suffer from the stress. So I think the 'hypervigilance of PTSD will definitely have a big impact on those living with a sufferer.
Algm, with regard to your questions, I think PTSD can make it so hard to give loving or calm responses. Because my father had PTSD I know how very tough it can be to deal with someone who is so often defensive, or agressive, even, because he sees threats and hostility where really none (or only a little of it) exists.
I don't really feel qualified to offer advice... just my thought is that to stay because you fear your partner would end his life if you left is not a situation that's bearable in the long run...
Seems to me that the two of you should communicate about this and you should get it more clear if he does or does not want you around. If he does AND if he can see or realize that he's making your life hard, and willing to try to find ways to improve, it may be easier for you to stay and be motivated to stay.
However if he says he wants you around but doesn't seem to work towards healing, I think it'd be very hard to stay in the relationship. Sorry to hear you're having to face this.
Freya