they're all dead (''we're all transparent here'') and the group can see them because they've gone mad.
that and they want the baby
Very Lost indeed. But he may have a point about them being 'dead' in a metaphorical sense. In the groups eyes these people are as good as dead, as they are yet to experience any real threat.
My theory, husband is aggressive towards wife, Rick intervenes. Husband seeks revenge. Causing a snowball effect within the walls of Alexandria.