>>281749075
There's a few possibilities here.
Death may have hoped that Asa would influence Yoru enough to the point where she'd abandon her original dream (which seemed quite close to happening until the nuclear invention).
Death Devil may have been willing to "lose" and step aside for such a "benevolent" War Devil.
It could also be that she needs this weapon for currently unknown purposes, like erasing another powerful Devil.
Also I'm not sure she really needed it to be "that" powerful, she seemed quite content with Asa weaponizing Denji when Denji was just a random scumbag to her.
In fact she may now be scheming to pit them against each other in another possible attempt to provoke the War Devil into turning Denji.