>>281749890
She was still trying to keep up her "Famine Devil" alibi at the time so saying that to Nayuta made sense. Also Denji was involved with that whole situation so admitting that she tried to kill them would've made Nayuta more hostile. Saying she just tried to turn Asa into a pawn calmed Nayuta down a bit.
Death turning people into a pawn seems like more of a last resort when she sees no other use for that person. She acknowledged that the ability has some unknown restrictions and that she prefers not to use it. In Fami's case it's probably just her being too retarded and knowing too much to be left to her own devices, not to mention she's dangerous lunatic despite her ineptitude.
Denji being a "knight in shining armor" to Asa during Falling Arc opened up the possibility of Death Devil guilt-tripping Asa into "saving" Denji and thus keeping them close.