Not a big fan of existentialism but I'm pretty sure it postulates that morality is subjective. So the question would not really be
< do "good people must do bad things sometimes"
but
> do good actions exist
idealism postulate that there are metaphysical forms of the our material reality, which are perfect and unaltered. Therefore material reality is the corruption of perfect metaphysical objects into the physical. At first I'd be tempted to say that it does apply. However if you look at this closely the question would be, I believe; >does morality exist as a physical object or metaphysical object?
If the former then it must be corrupted, as an physical object (assuming you are in the idealist mindset) by its material form.
If the latter then it cannot be corrupted so morality is objective but since people (who apply it) are material they cannot be the elite of moral because of their material corruption hence the can do bad thing even if the are good.