Itachi said you have to kill your best friend to awaken the MS, but I think you just have to have had your best friend die. As evidenced by the fact that Itachi's death awakened Sasuke's MS even though Sasuke didn't really kill Itachi himself.

For Itachi, that was Shisui. For Sasuke, that was Itachi. For Obito, it was Rin. For Kakashi, it was Obito.

But with Kakashi, he didn't KNOW about it until he found the secret Uchiha tablet after Sasuke left the village. So even though Kakashi met the requirements a long time ago, he wasn't able to use the MS until the tablet taught him how.