I know that ninja were used for espionage and other covert missions in 15th century Japan, so how could they coexist in a society based around fighting with honor. As far as I know the samurai were governed by an extreme code of honor and there were honorable and dishonorable ways of fighting. Ninja were used by different groups to gain an edge on their opponents. They did so using espionage and stealth, but in a society were samurai were dominant in warfare how was this perceived?

Were they seen as a necessary evil or were ninja tactics regarded as honorable? Could it also be that during the 15th century Japans warrior culture had evolved so that all became fair in war?

Please let me know if I am making any wrong assumptions