We will probably either have to torture him or offer him food and medicine for information. Mainly just something that will influence AJ as a character.
Next after getting information we can either kill him or use him as trade, again to influence AJ and the group will probably have different opinions on killing him or not.
What could happen Abel if alive will probably try to escape when heading to the delta and if you told Aj killing Marlon was okay he'll kill him, if you told Aj killing Marlon was wrong he'll let Abel escape of course Abel doesn't make far before causing more pain to himself.
The delta of course wouldn't what a injured person so they caught you and kill him if you killed Abel you still get caught.