The difference between اسم الفاعل and الصفة المشبهة is that the latter denotes a permanent quality while the former doesn't. Thus, جميل means "beautiful", which is a permanent quality, while قاتل means "killing", which is not a permanent quality.
Another example: سامع and سميع. The latter means "(permanently/always) listening", and is thus an exclusive attribute of God, while سامع can be applied to anyone who happens to be listening.