Maine Sen. Susan Collins said she fully expected to come around and support Donald Trump in the general election--but attacking Army Capt. Humayun Khan and his family was the "tipping point" that made her decide she'd never be able to get on board.

"It was inexplicable to me that anyone, much less a presidential candidate, would not honor the sacrifice and empathize with a family who lost the son in war," Collins said Sunday on CBS' "Face the Nation." "Instead, he attacked them and attacked their religion."

Collins announced in a Washington Post op-ed last week that she would not be voting for Trump, the latest in a series of high-profile GOP defections.

"It was still a difficult decision for me, because I'm a lifelong Republican and I wanted to and expected to be able to support our party's nominee," she said. "But the barrage of cruel comments and the attacks on people who are vulnerable and unable to fight back really troubled me."

Responding to those who say that by denying Trump her endorsement she's helping to elect Clinton, Collins said she has no intention of helping or voting for the Democratic nominee.

"If I were helping to elect Hillary Clinton, I would have endorsed her, I'd be working for her and I'd be voting for her," she said. "I'm not doing any of those things."

Instead, Collins said she's taking a look at the Libertarian Party ticket, which is headed by former New Mexico Gov. Gary Johnson and former Massachusetts Gov. Bill Weld.

"If Bill Weld were the head of that ticket, it would be an easier choice for me because I know him well and respect him a great deal," she said. "I may go that route, or I may end up writing in the name of the person I think is best qualified to be our next president."

Collins said she let her Republican colleagues know before she backed out of supporting Trump, including Republican National Committee Chairman Reince Priebus and Senate Majority Leader Mitch McConnell.

"I think they understood that this was a decision that I, like many Americans, really struggled with," she said.