Around the era of contact, if a tribe killed a member of another nation that came to that nation carrying a calumet (peace pipe), this would be like a breach of the Geneva convention. Neighboring nations would regard that nation as the lowest kinds of dogs or snakes in the grass. The offended nation would endeavor to obliterate the nation that killed their diplomat, & neighboring nations would likely ally themselves with the offended nation to take advantage of the losses the offending nation would suffer as well as to cultivate potential future cooperation with allied tribes.

As for "killing civilians," in the contact era if your relatives were killed by members of another nation, you were expected to take vengeance on members of that nation until the chiefs of your nations bury the hatchet (figuratively or literally.) Not everyone may be killed though; it was common to capture individuals & if found suitable, you may have adopted someone captured from another tribe to become the relative your family had lost.

Much of the balances between nations were disrupted with European contact. The post contact period was very bloody, possibly due to the vast amount of deaths that befell tribes after epidemics leading to the reckoning of more conflict. It was very common in the recorded period for hunting grounds to constantly change hands in conflict or somebody deciding to murder a traveler to steal his stuff. Cycles of vengeance killing were recorded by the Europeans indicate this was common during their presence in the land though I suspect it may have been less common before their arrivals. The post-contact generations of several tribes may have looked at their world as post-apocalyptic one, which may have led to conflict more often due the stresses of seeing your way of life destroyed. Drunkenness of a tribesman was often recorded by whites as what led to an instigation of conflict between tribes, but these sources should be taken with a grain of salt.

I'm speaking from what I've read over the years which is mostly specific to the Great Lakes, Mississippi river & Ohio River regions from 1650 - 1800.