In his post, which has since been deleted, he also repeatedly named the alleged gunman.
The post also drew criticism from fellow Hamilton city councillors, with councillor Dave Macpherson calling Casson's views a "stain" on council.
On Tuesday, Casson laid a complaint with the council over a series of public posts and comments made in response by councillor Dave Macpherson.
The two councillors met Wednesday morning with King, and since then Casson has withdrawn the complaint.
Mayor King says the decision shows it is time to move forward and for councillors to resume working together for the good of the city.
"Councillor Casson has unreservedly apologised for his comments," King said.
"I have no doubt Casson has learned a lot in the past few days and has recognised the effect which his words have had on people in grief.
"There are lessons for us all here.
"I have once again emphasised to my council the importance of respect, understanding and empathy for all of our community.
"As elected representatives we need to be leaders for our city.
"These are exciting and important times for Hamilton and that is whee we will, and must, focus our attention."
Casson was not the only city councillor in the spotlight, with councillor Siggi Henry also causing a storm for wearing an anti-vaccine T-shirt to an autism awareness event.
Last Friday, 300 people attended the Light Up The Lake fundraising walk around Hamilton Lake, the event was part of World Autism Awareness Day.