"Each reaction ... he sits in his cell in the smug comfort of knowing he has impacted on everyday Kiwi life."
In a text message, Casson told the Herald the reason behind his resignation was because he was pursuing other business interests.
"As it is a personal employment matter I will not be discussing it further having respect for Immigration NZ," he wrote.
In his Facebook post, Casson advised Kiwis against mourning for the victims of the Christchurch attacks - saying that in doing so, they're letting the alleged gunman "win".
"Let's move on as a nation, never forget or forgive but strive to live as normal," the post concluded.
The deleted post wasn't Cassons' first public criticism of the way officials and the public reacted in the aftermath of the shootings.
A post on the Facebook page published the day after the incident discussed a vigil at Hamilton's Claudelands Park - which Casson made clear he would not attend.
"I am sure it will be safe but I do not want this to turn into a political showcase either, it is not the time for that sort of stuff," he wrote.
"The most effective way of showing such acts of terrorism that we are strong and united is to carry on with daily life normally."