Your description of the problem is a little ambiguous. Two questions: is the unit cooling at all, and by 'spins easily' do you mean that the fan runs normally under its own power? The hum could be a compressor that won't fully start, or a compressor that is stuck on. In the former case the unit won't cool and in the latter it will, but the compressor will remain on after the fan turns off and thus seem to hum.
If the unit is cooling and the only way you can stop the hum is to turn of the circuit breaker then it sounds like your compressor is stuck on (this can happen if you have a bad relay control board or compressor control relay.) If either the fan or compressor (or both) won't start and you are seeing no cooling then the start capacitor is certainly suspect.