Can I say that only in some cultures is/was suicide seen as a way to preserve honour (e.g. seppuku) even in other cultures during this time it could still be seen as a negative act.

Alexander Murray looks at suicide in the middle ages in his eponymous book series. /u/sunagainstgold tackles that topic here

Academically though it could be said that French sociologist Émile Durkheim's 1897 work Fr: "Lé Suicide" Eng: "Suicide: A Study in Sociology" helped to shift this current view into the mainstream as it really got to the nitty gritty of why certain people experience suicidal ideation and commit the act itself.

Probably the main reason suicide was seen in the west as negative has to do with Christianity. There are only seven suicides mentioned in the Bible, but for Christianity the most important is that of Judas Iscariot. Christian philosophy says that Judas might have gone to heaven if he had repented his crime and not committed suicide. St Augustine importantly linked suicide to the biblical commandment "Thou Shall Not Kill," to include killing oneself. St Augustine relies heavily on Plato's Phaedo arguement that one does not own one's life, that it is the property of God. However, it wasn't really until the period around 600 AD that we see suicide become a crime.