I think Minsc may be explained by the fact that alignment is on a scale (the game does not deal with this at all). As you commit acts large and small that are far from your current alignment you begin to shift toward another one of the alignments. Perhaps partially due to his head injury, and later his experiences with Dynaheir, it makes sense that the original sane and happy Minsc shifted away from neutrality to a more chaotic nature.
Some of Kivan's behavior could also be explained by a shift in progress, given his own personal trauma and need for vengeance.
I don't really see how we can judge alignment of NPC in BG1 : if the alignement was not written on the stat sheet, you would have almost no clue of what it should be ( we know almost nothing about them and they comment and banter very rarely)
While evaluating their alignments we also take in consideration their voices, their portraits.
For example, how can we say Xan's personality and alignment are not understandable? "We are doomed" + his voice-over acting + the circumstances of meeting him + his banters are enough to understand Xan.
There're plenty of drawings, stories, fanfiction on BG1 NPCs that, in my opinion, serve as a testament that BG1 personalities are quite interesting.
Also, there's a BG1 NPC mod that combines both understanding of BG1 NPCs and developing their personalities even further.
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Some of Kivan's behavior could also be explained by a shift in progress, given his own personal trauma and need for vengeance.
While evaluating their alignments we also take in consideration their voices, their portraits.
For example, how can we say Xan's personality and alignment are not understandable? "We are doomed" + his voice-over acting + the circumstances of meeting him + his banters are enough to understand Xan.
There're plenty of drawings, stories, fanfiction on BG1 NPCs that, in my opinion, serve as a testament that BG1 personalities are quite interesting.
Also, there's a BG1 NPC mod that combines both understanding of BG1 NPCs and developing their personalities even further.