You have to feel the pain of loss of losing your loved one. So if you were to have the sharingan and wanted the mangekyou, killing your loved one wouldn't make you get the mangekyou because you wouldn't feel the pain of loss. However, I find it interesting that you go blind from your use of mangekyou, because you can simply take the dead loved one's eyes and get the EMS and, later, Rinnegan.