Hi I'm a mother of 4 but it is my 5yr old need help with?

my nice baby girl has gone she has killed are baby cat and she is knot sorry for it . she new what she was doing has told me she want to have a party that are cat is gone called criss line waiting for tueday for a doctor to see her but i need to now why she did this and if there is hope for her to grow up to be ok after this my other kids never did this and she is the baby we have a mother cat and a lg dog she told me she can't do nothing to them they bite ???? she is out of control yes i now that but like most of you say i have spacked her and did all the punchments in the world be for this to stop all the other behaver stufff she dose but this one I did not now what to do did not want to mke it worrse then it was just want to hear from other people that have had this happen dont need to hear I'm bad feel that why now thanks just looking for people that have dealt with this befor ok thank you