The notion that death can be life-giving doesn’t appear to make philosophical sense, Thomas notes (ST III, 50, 6). Death is a privation of life, and a privation doesn’t have power to act. Therefore, no death, including Christ’s, has power to give life.
Thomas admits that Christ’s death (as opposed to His Passion, His sufferings) does not bring life by way of merit. But it does have life-giving power by virtue of “causality.” This is true only because the Son is not separated from His human flesh even at death, which means that “whatever befell Christ’s flesh, even when the soul was departed, was conducive to salvation in virtue of the Godhead united.”
There’s a necessary symmetry here. Effects resemble causes. Since Christ’s death causes as a kind of privation, the effect of that death must also be privative rather than positive. Christ’s death thus effects “the removal of the obstacles to our salvation,” namely, “the death of the soul and of the body.” By removing those obstacles, Christ’s death becomes life-giving, and death is swallowed in victory. Directly, the privative cause (death) produces a privative effect (removal of obstacles); indirectly and at a second remove, Christ’s death gives life. Privation of death removes privation.
Once again, it’s apparent how important it is to Thomas that the Godhead remains united to the flesh even in death. Without that union, Christ’s death would not overcome death, would not be life-giving.