The Vermont senator told CNN's Jake Tapper the former secretary of state is too quick to "rush in" and remove dictators and he criticized Clinton's approaches to Iraq, Libya and Syria.
"I worry about that, yeah, I do. I think her support for the war in Iraq was not just an aberration," Sanders said of Clinton's vote to authorize the Iraq War, in the interview that aired on "State of the Union."
"I think that her willingness to kind of push President (Barack) Obama to overthrow (Libyan leader Moammar) Gaddafi and lead to the kind of instability that we're seeing now in Libya -- not inconsistent with her other views on Syria, where she wants a no-fly zone, which I think can suck us into never-ending conflict in that area," he said.
Sanders' sharp critiques come as Clinton is on the cusp of being declared the presumptive Democratic nominee, through a combination of pledged and superdelegates.
Speaking about Iraq's Saddam Hussein and Gaddafi, Sanders said: "These are terrible, brutal dictators. But our job is to think what happens the day after these people are removed -- the kind of instability that occurs."
"The world would be better off with these dictators out of power, but there are ways to get them out of power without causing mass instability and the deaths of many, many, many hundreds of thousands of people," Sanders said.
He conceded that "no, it's not that easy, but you bring the entire world together."
Sanders was also sharply critical of the Clinton Foundation -- the Clinton family's charitable organization -- for accepting millions of dollars in donations from countries like Saudi Arabia during her tenure as America's top diplomat.
"Do I have any problems when a sitting secretary of state and a foundation run by her husband collect many millions of dollars from foreign governments which are dictatorships?" Sanders said. "You don't have a lot of civil liberties or democratic rights in Saudi Arabia. You don't have a lot of respect there for opposition points of view, for gay rights, for women's rights."
"Yes," he said. "Do I have a problem with that? Yeah, I do."
Asked if he thinks those donations create the appearance of a conflict of interest, Sanders said: "Yeah, I do. I do."