For all we know,he is in great financial need.
Now this is speculation based on indications that we have. He sent a letter to his patients saying that he suspends medical practice permanently. I have this letter. Why would he say that if he was going to work for MJ for 1-2 years max? The man relies on his job to cope with rents,facilities,child support, lifetime expenses.
Most likely he was promised a big amount to get Michael out of the way. So after he does a little jail time (or not) he will be free to go with all his finances settled for life. We all know Michael had powerful enemies who wanted to seize his assets and for them he'd better be off dead.
Still, I haven't ruled out the possibility of not even being at MJ's house while someone else .
All this to be proven in court.