Drug users

I've never even touched drugs.  My soon to be xhusband however did.  I knew he had a problem before the military and once he got out that when he started using again.  From what I've heard it's possible he's been using prescription meds and alcohol.  My question is does a person in this situation realize how much harm they are doing to the family?  How leaving is for something like that is tearing at us.  Even though I'm going forward with this divorce after all the cheating and lying and name calling with manipulation I still do care for that human being that once lived,  he still thy father of this unborn child.  I'm trying to wrap my head around it however I can't seem to understand it.