The ghosts will act as ICly as possible under the circumstances - so it entirely depends on the original character. They can't stop themselves from sapping health from their host, but if they were a good person who loved the host in return, they might regret what's happening, and even offer themselves up for death/ask their host to kill them. They might not figure out what's going on, or they might regret what's happening but still want to live. They could even have no regrets at all. We don't choose who we love, after all, and sometimes we love some pretty shady characters.