Opposite of benefit of the doubt?

Solution 1:

"Assume the worst" is the best idiom I can come up with that conveys the opposite of "benefit of the doubt".

Sam: I cannot marry you because since I don't know you, I have to assume the worst."

Solution 2:

"Assume the worst" sounds perfect to me, but in that specific situation, I would say something more like "I don't know you, I'd rather be safe than sorry." Being rejected will hurt either way, atleast this has an "I want to be safe" sound, rather then "you might be bad."

Solution 3:

The idiom "prevention is better than cure" could also be an opposite for "benefit of the doubt".

From dictionary.cambridge.org, prevention is better than cure UK saying (US an ounce of prevention is worth a pound of cure)

It is better to stop something bad from happening than it is to deal with it after it has happened.

https://dictionary.cambridge.org/us/dictionary/english/prevention-is-better-than-cure