When Allah is so Raheem and Kareem why didn't He stop the harsh behavior of Yazid and his army on Imam Hussain and his family?
Allah (SWT) does not enforce justice in this life stopping people from doing wrong, but He gives freedom and leaves human beings to select their own options, whether good or bad, although He guides and helps them to do good, that is why He did not stop Cane from killing his brother Able and did not stop the criminals who killed prophets like Yahya (AS) and did not stop Ibn Muljam from killing Imam Ali (AS). That is the Justice of Allah which gives every one the result of his own actions.
Wassalam.