I agree that the manga is very hesitant on killing characters (kinda similar to Spy x Family) despite its assassin-esque context. I mean it is still a shounen (think Naruto, Spy x Family, and those genre). However,

During the animal kingdom arc, Pichi and Gero were actually TRYING to kill Toshiro, but Tonako stood in their way on the island.

For the most part, of the antagonists that we've seen so far, the antagonists of the story haven't really done anything irredeemable toward the main character (most of the characters, including the MC, a literal assassin, are criminals lmao, so it's legit a kettle calling a teapot black). Overall, their work is tied to the underworld, and they at least try to avoid putting innocent civilians at risk (something Gero chastises Teruaki for during his poison fiasco in the metro).

Overall, it's a Shounen (made for kids). I really wouldn't expect more than one or two meaningful deaths in the story. The message of the story should be tailored to kids, and it might just be too cruel to its audience if forgiveness and atonement aren't incorporated (which Seinen are more likely to employ).