It seems to me that suicide is prohibited or frowned upon in many of the world's religions and cultures, including the Abrahamic religions, Buddhism and Hinduism, notwithstanding some exceptions such as martyrdom.
Do we have any evidence pointing to this belief appearing in one place and then being propagated, or did it independently appear in multiple locations? Is there a benefit in discouraging suicide that helped spread this view? What is the earliest written record condemning suicide we know of? Supposing there is an inkling of truth to stories of mass assisted suicide recounted by Josephus in the sieges of Jotapata and Masada, do we know of any stories of assisted suicides prior to these examples?
In the Christian Europe suicide began to be viewed as a sin in the fifth century, when Augustine argued against suicide in his City of God. His argument was that it broke the 6th commandment "Thou shalt not kill". He also argued that, because it is God who decides when one dies, killing one's self is denying his will. Subsequent church councils banned church burials for suicides, and in the second half of the middle ages bodies started to be desecrated. In the 13th century many places started confiscating the belongings of suicides as well.
Before this in Europe during the antiquity there were arguments about how acceptable it was. Some argued that it was acceptable under certain circumstances. But these arguments and beliefs differed from that of medieval and early modern Europe because the former tended to believe that it was heroic in some cases and dumb in others while the latter usually believed that it was always a sin no matter what the circumstances were. Their beliefs against it aren't, as far as I know, that connected so it is likely an independent belief.