Spouse's hopelessness and misery

My spouse has ADHD as well as other things (anxiety, depression, ??).  He is a full-time caregiver for his elderly parents.  An example of what this entails is that his mother, who has Alzheimer's disease, needs assistance with toileting, and she is resistant to the diaper and clothing changes.  Meanwhile, his father, who also has health issues but is mentally competent, is, like my husband, depressed and feels hopeless.

I would like to help them out.  I have offered assistance and information several times.  Yesterday, on the last day of a trip to see one of our daughters, a trip that I arranged not only so that I could see my daughter but so that my husband could have a break from caregiving, I once again asked my husband if there is anything I can do to help.  And he said once again, as he has done a few times this year, "Shoot them."  

I know this is venting.  I know my husband won't harm himself or his parents.  But he is suffering, his parents are suffering, and he and his father are, in the fashion of depressed people everywhere, both uninterested in doing things on their own and resistant to receiving help from other people.

Any thoughts?