Twelve people were killed and 59 injured when suspect James Holmes opened fire in a packed Colorado movie theatre.
It has been reported that Holmes called himself The Joker - the crazed villain played by Ledger in 2008's The Dark Knight - and even dyed his hair red in homage to the clown character, and Kim Ledger has been left disturbed by the news.
He has also called for a tightening of firearms laws in the U.S. and tells Australia's Herald Sun, "It's terrible - the whole circumstances. It's diabolical. I am appalled. The guy's obviously a nut case. He has obviously lost it.
"I think what America should be doing is restricting the availability of ammunition or revisiting their gun laws - that's what they should be doing.
"We can't blame Heath or the character. It's (The Joker) fictitious. I don't know what this does to the character. I think that's the least of my worries."
Ledger, who died in 2008 during post-production of The Dark Knight, won a posthumous Academy Award for his performance.