'Cruel ' usually refers to someone who is physically brutal or violent. However a person can be emotionally cruel towards others as well. "Ruthless" refers to someone who is cruel and will do anything to achieve his or her aims, but not necessarily by violent means. He or she could use the judicial system to bankrupt an enemy for example.

"Cruel " describes an act or person that is both brutal and very violent. "Ruthless" describes an act or person that is cold or cruel, but this act or person need not be violent. A ruthless person believes that the ends justifies and is willing to do anything in his/her power to achieve his/her objective. The motivation--to achieve an ambition--must be kept in mind when understanding the word "ruthless". On the other hand, "Cruel" does not include any motivation--it merely describes the kind of violence.( Causing suffering; painful.)