Did Mr. Harvey kill Ruth? Is that how Susie comes to use her body?
No, Harvey did not kill Ruth. We don't really ever find out why Susie was able to use Ruth's body. It was probably because Ruth felt close to Susie and allowed it to happen and it was an intensely emotional moment.
No, Mr harvey doesnt kill her, the only reason Susie is able to get into her body is because Ruth actually wants to see heaven and is really affected by stuff like death. She feels at home at heaven. Ruth even feels Susie dying, so she has a connection to dead people.