Putin saw an opportunity with a weakened United States with a puppet president and exploited it. They were expecting to waltz in as easily as they took over Crimea in 2014, and followed by their separatist wars in the Donbass which Ukraine largely recognized in said agreements.
Like Hitler, after gaining the Sudetenland by agreement in 1938, then decided to go for the whole of Czechoslovakia just five months later, Putin decided to make a similar move on Ukraine, not because the west was particularly supportive of the sovereignty of either county but because the tyrant next door believed that they would not be.
The only difference is that Hitler's calculation was correct and Putin's was not. It had little to do with the accords signed in either Munich or Minsk and everything to do with the perception of opportunity and easy conquest.
“Your points are noted, but nobody forced Putin to unleash the brutal invasion of a neighbor that posed no threat to Russia.”
I think the US lost that moral ground virtually the day the Cold War ended.