Hi!

Is there a way to make neutral characters good or evil? (Items, traps, foes,...) Preferably evil, because I want to class change to ninja!

No.

If you want a ninja, the character needs to be good or evil, unless you want to back transfer to Wizardry 1 (possible on the Apple 2 version, not on the DOS collection, and has the drawback that the character will be reduced to level 1 with level 1 stats and no spells when transferred back to Wizardry 5).

With that said, ninja are honestly disappointing in the early Wizardries:
* Unarmed combat isn't that strong, and in particular, doesn't seem to scale.
* Unarmored AC improves too slowly (4 AC at level 13), so don't expect that to be worthwhile.
* Ninja are good at disarming traps, but not inspection. A thief's inspect chance is AGI * 6 (capped at 95), while a ninja's is only AGI * 4 (which can at best be 72 except in the SNES version which changes the rule for stat caps); hence, you are going to need someone who can cast CALFO if you don't have a thief.
* In Wizardry 5, lords get better equipment than ninja.

Edit: Also worth noting that the item that allows the class change is rare in Wizardry 1; I think it's a bit easier to get ahold of it in Wizardry 2 (with another item that has a 1 in 3 chance of changing the user into a ninja that isn't in Wizardry 1).

Thanks, so I need to make a new thief, who is evil. I know, but this is no option for me, I´m playing the SNES-version. Better graphics, you know?

There's also one rather significant gameplay quirk of this particular version: The stat cap is not 18. Instead, each stat can be increased up to 10 points past the racial base. For example, a Dwarf can get Strength and Vitality higher than any race, resulting in (slightly) increased damage as well as more HP per level, but Agility will be limited; on the other hand, a Gnome can reach 20 Agility, which is enough to always act before the enemies (unless there's a bug; the SFC Wizardry 1+2+3 has a bug here).

In particular, a Hobbit Ninja with 22 AGI (possible in the SNES version) will have an 88% chance of inspecting traps. (Compare that to a Thief, who will have a 95% chance at 16 AGI.)

By the way, this stat cap change has been adopted by Japanese Wizardry games as well, which in turn means it can be found in Elminage Gothic (great game, by the way, though it does get rather brutal, but then again, so does Wizardry 5) and Stranger of Sword City (though the cap is base + 20 in SoSC).