I personally think it’s that crying girl . She was acting at the funeral. I think she killed her because the dead girl had some affair with her boyfriend(the guy who tried to console her) these two girls where friends so she felt betrayed about this and killed her. Because we see this dead girl as a mischievous girl who doesn’t give a damn about anything like not paying for the ice cream and letting a random stranger into her house and doing stuff so she could very well have affair without any guilt towards her friend and she also mentioned to the male lead that she was staying with her friend and she left recently. Maybe the crying girl left the house because of this reason and this make perfect sense of how someone can sneak inside a house without anyone noticing because she knows the passcode of the house or know that the other window lock is broken(shown in ep-5) so she can enter through that also and did it.
Because we see this dead girl as a mischievous girl who doesn’t give a damn about anything like not paying for the ice cream and letting a random stranger into her house and doing stuff so she could very well have affair without any guilt towards her friend and she also mentioned to the male lead that she was staying with her friend and she left recently. Maybe the crying girl left the house because of this reason and this make perfect sense of how someone can sneak inside a house without anyone noticing because she knows the passcode of the house or know that the other window lock is broken(shown in ep-5) so she can enter through that also and did it.