Because he administered drugs that he wasn't supposed to to a patient he shouldn't have.
He misadministered drugs because he's an ethically challenged doctor bought by an emotionally disturbed celebrity. A celebrity who wanted unrestrained access to any drug he wanted. Why are you taking away MJ's culpability in this? He was a disturbed man spent his whole life finding unethical doctors to mutilate his face and his skin, give him unnecessary and illegal drugs, and do what he wanted when he wanted.
You keep saying "murder". If that makes you feel better. In mind "murder" means someone took actions to kill you, wanted you dead, and ended your life on purpose. Clearly Conrad didn't want that result. But if you need him to be a murderer to make MJ's sad death simpler, by all means go ahead.