[noirscape]

As a native dutch speaker here's how I would translate it:

"We shouldn't add errors to software out of negligence. They should be added systematically and carefully."

[amelius]

Yes, and so always attribute to malice that which is adequately explained by stupidity.

[OnlyMortal]

Literal translation:

“That must we systematically and with care do”.

[jbverschoor]

I’d translate it more like out of carelessness/arrogance

[vidarh]

Nonchalance is in English dictionaries.