For a period of about 5 years Hitler caused the agonizing deaths of 10 million people. So [human] justice would have him experience all the pain he caused. Which may take about a million years.
The second million years of agony would just be gratuitous.
Wherefore in order that the happiness of the saints may be more delightful to them and that they may render more copious thanks to God for it, they are allowed to see perfectly the sufferings of the damned.
On the first Sunday after the terrible events of 9/11, the preacher at my church made a passing reference to this and I'll confess that it brought many of us comfort knowing that, from Heaven, the redeemed in Christ would indeed see the Islamist terrorists enduring unspeakable pain for all of eternity.
I would surmise that we likely differ on this and perhaps quite strongly. In fact there's a school of thought that Aquinas was giving a pass to sadism (or at least schadenfreude). To the contrary, and from my perspective, the saints in this scenario aren't "delighting" in the actual suffering of the damned as such but do indeed get unspeakable joy in witnessing the Righteous Justice of God meted out for time everlasting.