Murder of Crows has a triggered ability that uses the word ‘may.’ This indicates that the ability is optional and you do not have to draw and discard a card when it resolves. Because it is optional, it also means that you will not receive a penalty if the ability is missed. In instances where this trigger is not dealt with, it is assumed that it did in fact resolve and its controller chose not to take advantage of it. In other words, the controller chose not to draw and discard.

One final thing to point out is that the entire ability is optional, not just part of it. If you choose to draw a card when it resolves, you must also discard a card. Additionally, you cannot choose to not draw and just discard.