no, remember all the "Freindly monster groups" you encountered?
if you leave them alone enough times, Evil characters will change to good. how many times, is apparently random, as some characters will change before others.
If you fight them enough times, Good Characters will turn Evil. again, it seems random.but, despite what it looks like, Evil is not necesarily bad. it just means the character is being selfish. To take something from the Wizardry 1 manual, Evil characters ... want to know what's in it for them. an evil character will walk an old lady across the street for a small fee.
while Good ones will go out of their way to do so, and Neutral will if they are headed in the same direction.
also, only Evil characters can be Ninjas, as only Good can be Lords. you just can't make a party from the adventurer's guild that includes both Good and Evil characters, but that does not mean you can't invite them to your party (hint, hint)
also, Good and Evil can change Good<->Evil either way, while Neutral is forever doomed to Neutral status.
I'll start with a caveat: I haven't played much of Wiz5. Most of my time in the early wizardries is spent in Wiz1. Also, I haven't played ANY on SNES.That being said, the cause of your "problem" is probably the same: in most of the early wizardries (1-3 & 5), "good" and "evil" characters can change alignments based on their treatment of "friendly" monsters. If "good" characters choose to fight "friendly" monsters, there is a chance that they will become "evil," and if "evil" characters choose NOT to fight "friendly" monsters, there is a chance that they will become "good." This is not a bug, and it's not even a bad thing. For example, you can utilize it to get an "evil" lord or a "good" ninja.
Neutral characters, by the way, do not change alignment, at least not as far as I have ever seen or heard.