weather or not the malach a mavet is necessary i don’t know, but i do know you can’t actually kill a person. ill try to explain what i mean but if you don’t understand there is a great podcast called the shmooz he discuses it.
you can’t even lift your hand without H’ allowing you. this is the whole reason why ican do tshuva from bein adam la chavero since anything i do is with the help of H’ he wouldn’t let someone get something bad to happen to them if they didn’t deserve it/ wasn’t best for them. so i am but a messenger (message ilustrated in megilat ester when Mordechai tells Esther that if she doesn’t rise up to the task someone else will) not why is it then a sin if i am carrying out the will oh H’? it is my intention, i wanted to cause harm and in the case of the city of refuge it is i am not careful/ conscious enough about those around me. for this reason when one changes his mind in complete tshuva the sin is erased since the physical harm doesn’t come from you.