As I understood the difference between "sin-" and "mem-":
I killed myself (I killed me)
Mi memmortigis la ŝafon.
I killed the sheep myself (I killed it on my own)
But someone told me that "sinmortigi" and "memmortigi" are actually the same! Really? Reta Vortaro says that "sinmortigo" = "memmortigo". Is that true, and should we avoid that?