The difference is that during the latter half of the Cold War, neither the US nor Soviet Union deployed the types of ABMs prohibited by the treaty, which would have increased the opposing countries dear of a first strike and increased the risk of nuclear war. While, frankly, I'm not convinced of the merits of the decision to pull out of the Treaty, it became substantially less relevant with the unravelling of the bipolar US/Soviet rivalry as the dominant geopolitical conflict, and there's at least an argument to be made that the US withdrawal and active pursuit of missile defense reduces the expected utility of new ballistic missile development by countries seeking them largely as a strategic tool against the US, giving the withdrawal a positive net deterrent effect in the non-bipolar world we now live in (there are also significant arguments that the net effect is not beneficial, and while I lean toward the latter conclusion, it's not an open-and-shut case.)

Withdrawing from the ABM Treaty is not analogous to murder.