Im muslim, i say go fk ISIS... disgrace to Islam.
They have taken the responsibility, but there is no clear statement from them. Is there any specific reason for the attack that took away the life of 300+ women, men and children. But prior notifications have been provided from the neighboring countries, but it wasn't taken seriously by the officials which is a big mistake.
You can check last edit in op, according to Sri Lankan's authority it was retaliation.