Both stupidity and malice happen. It can often be difficult to tell the difference as we arent as objective as we think we are.

However, accidentaly attributing stupidity to what is malice is generally not too bad. If its malice it will happen again and you can revise your opinion.

Accidentally attribute malice to what is stupidity is an easy way to start grudges. This can blow back on you and turn someone who just made a mistake into someone who does actually hate you, and make third party observers think you are unreasonable.

So erring on the side of assuming stupidity is generally a good call.