"That's a butcher. That's a butcher," Trump said of Assad. "So I felt we had to do something about it."
The stark language is the most blunt terms Trump has used to describe the Syrian president, who is accused of war crimes in an attempt to hold onto power during a prolonged civil war. In the past, Trump has called Assad "evil" and "an animal."
Speaking of the missile strikes he authorized against the base that carried out the chemical attack, Trump later added: "I have absolutely no doubt we did the right thing."
Trump's top aides said earlier this month that the President was moved by the photos and video of children suffering from the effects of the chemical attack and that those pictures, in a way, compelled him to take action against the Syrian president.
On Wednesday, standing with NATO Secretary General Jens Stoltenberg, Trump echoed that sentiment.
"Young children dying, babies dying, fathers holding children in their arms that were dead, dead children," Trump said. "There can't be a worse sight, and it shouldn't be allowed."
Earlier, Russia vetoed a UN resolution condemning the killings,
believed to have been carried out with sarin gas, and called on Moscow ally Syria to cooperate with an international investigation of events on the ground.
It was the eighth Russian veto of a resolution on Syria throughout the course of its civil war.
US Ambassador to the UN Nikki Haley told the council that "with its veto, Russia said no to accountability, Russia said no to cooperating with the UN investigation, Russia said no to helping keep peace in Syria, Russia chose to side with (Syrian President Bashar al-) Assad, even as rest of the world, even the Arab world, comes together to condemn the murderous regime."
The resolution would have compelled the Syrian government to provide helicopter pilots for interviews with international inspectors and flight logs that could provide evidence of flights in the air over the city where the gas attack took place.
Russia's veto came just hours after US Secretary of State Rex Tillerson huddled with Russian President Vladimir Putin
for two hours in Moscow. Diplomats said Russia came close to proposing its own resolution, which would have included language condemning the US missile strikes in Syria last Thursday.
China, which in the past has vetoed several resolutions along with Russia, abstained on this one -- a move welcomed by Trump at a White House news conference.
The Chinese ambassador to the UN later said it was regrettable that agreement could not be reached. Bolivia also voted no, but the South American country does not have veto power like Russia.