I have fostered. I 'disapproved' of the mother's lifestyle enormously. I also felt a great deal of compassion towards her. The bad choices she made didn't take place in a vacuum. She was a hurt human being who was expressing that hurt in a completely dysfunctional way that was in turn hurting her child. Hence my involvement.
That child was too young for me to need to explain about her mother. However, I had it all prepared in readiness because it is vital that you can explain without judgement. It's not that dissimilar to when parents split up and they have to explain why that has happened without making the child think it's their fault.
Every child, perhaps moreso if adopted/fostered, wants to know where they come from. If you badmouth the parent, you are, essentially, encouraging a child to think that they are the same, since they are a product of that parent.
It is far better to remain neutral, or, if at all possible, find something positive about that parent's personality/abilities, even if in some harrowing cases it's just that they had a good sense of humour or were good at drawing. You talk about the parent's dysfunctional lifestyles or poor parenting in terms of choices (so that the child doesn't feel it is inevitable that they will be the same) but explain how those choices can be affected by experience (e.g. brought up by abusive parents) and sometimes just sheer random bad luck (e.g. accident or illness).
I think the best analogy for this is like making a cake. Sometimes the ingredients (intelligence, wit, sense of fun, ability at drawing, etc) can be of the highest quality. But if the recipe (life) isn't put together well, you can sometimes end up with a mess.