Speaking in front of his Orlando home, Eric Paddock told reporters he was having difficulty processing the "horror story" that unfurled on Sunday when his brother gunned down 58 people and injured more than 500.
Mr Paddock said there were "no types of clue" as to why the retired accountant opened fire on revellers of the Route 91 Harvest festival from the 32nd floor of his Mandalay Bay hotel room before killing himself.
"I hope they find there's a tumour in his head or something, if they don't then we're all in trouble," he told CBS News. "This is what everyone's so scared about right now."
He added: "Something had to happen to Steve - I'm not even trying to excuse it - but something happened that drove him into the pit of hell and he did this.
"Whatever happened to him, it happened over the course of months."
Mr Paddock added that his family was finding it difficult to cope with the news, saying "how the hell did this happen?"
"My son is just crushed by this and it has nothing to do with him. I'm an old school guy, this crying thing is not my normal gig, I cant even process this really," he said, as tears rolled down his face.
"Steve took care of the people he loved. He helped make me and my family wealthy. He's the reason I was able to retire. This is the Steve we know, we knew. The people he loved and took care of."
He also said he would be ready to do anything to help the victims, saying his "heart is torn, destroyed" and that he would even allow people to "spit on his grave" if that would help them.
Authorities are still scrambling to uncover Paddock's motive for the cold-blooded assault.
Las Vegas sheriff Joseph Lombardo warned he may have been "radicalised unbeknownst to us."
He added the way Paddock had meticulously planned the massacre was "troublesome", but said he was “absolutely” confident authorities will find out what motivated him.
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