He also said such accusations may increase ahead of the Lok Sabha polls as Singh had recently joined politics. (Agencies)
"There is no principle. It is a no holds barred attack. Such things will happen more as we approach the elections. May be this is an attempt to put pressure," he said when asked whether it was proper for Singh to criticise Shinde's style of functioning.
Pillai said it was not good for a bureaucrat to level allegations against his former boss, especially after retirement.
"It was not proper. But he (Singh) has joined politics. So, it is totally a different ball game now. This is a very complex situation," he said.
The former Home Secretary, who had worked closely with then Home Minister P Chidambaram when the country's internal security apparatus underwent a change, said the Home Ministry was a "very sensitive" ministry and one should be careful even after retirement.
In a scathing attack, Singh had said Shinde was not fit to be the Home Minister and Chidambaram was "100 times better" than him.
Singh had claimed that Shinde had allegedly interfered in the transfers and postings in the Delhi Police and accused the minister of giving wrong information about the US helping India in bringing back underworld don Dawood Ibrahim.
Singh joined BJP on December 13 and may contest the Lok Sabha polls from Bihar.
He also said such accusations may increase ahead of the Lok Sabha polls as Singh had recently joined politics.