In U.S. Criminal law, there is a popular cultural summation of the three aspects of a crime.

These three aspects are means, motive, and opportunity ---and together, they are investigated prior to establishing and determining guilt.

Respectively, they refer to: the ability of the defendant to commit the crime (means), the reason the defendant felt the need to commit the crime (motive), and whether or not the defendant had the chance to commit the crime (opportunity). 

Opportunity is most often disproved by use of an alibi, which can prove the accused was not able to commit the crime as he or she did not have the correct set of circumstances to commit the crime as it occurred. 

Motive is not an element of many crimes, but proving motive can often make it easier to convince a jury of the elements that must be proved for a conviction.

So, the motive behind the actions is very important ...though it is often difficult to prove.  It is nonetheless important ...and if the action can be proven, the burden of proof should often be placed upon the one who had acted, or not acted, to explain their own motives for action or inaction.