There's a considerable difference between never ratifying a treaty (in which case, it is never in force), and ratifying it, having it in force for nearly three decades, and then using the mechanism that the agreement itself provides to trigger its termination.
30 years of doing good doesn't count for very much if you then do something bad. If a 31 year old kills someone they can't use the prior 30 years of "not committing any murders" as evidence that they're actually a decent person.
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.
The name is a bit confusing it actually just limits the defensive deployment of anti-ballistic missile bases designed to shoot down ballistic missiles it doesn't do anything to limit ballistic missiles like the name kind of implies. It wasn't really preventing anything and more just cranking up the consequences of nuclear war (could argue that that decreases the probability of it happening by taking away the ability to shield yourself effectively).
ABMs situated after the boost phase were kind of pointless soon after their invention anyways because the MIRV (multiple independent reentry vehicle) warheads + decoys could easily overwhelm them.
Those 30 years included the last couple decades of the cold war. There was clearly some benefit to having an arms reduction treaty in place while we were still staring down the Soviets.
