The soul ahead of him has committed grave deeds and tainted their soul pitch black with sin. Lucifer finds this intriguing and decides to offer them a place amongst two other grim reapers, making them the third. To him, grim reapers are more like playtoys of his, sinners he's imbued power demons into and tasked them to go meet a daily quota of sinners' life reaped, no matter how it happens. He doesn't care whether a grim reaper kills a sinner for their soul, its all just a part of his game.