Sumire does not understand where these rumors came from, but she knew nothing would come out of denying it.
This person has already chosen to believe that. Nothing she says will convince him. Even if she were to present the facts right in front of him, the situation would remain the same. Humans are stubborn creatures like that. Once they choose to believe in something, they will firmly do so, and nothing, not even the truth, will sway them.
He already thinks she killed Ru.
If they think that way, this person and all the others may maybe she ought to accept it. There is no use in saying anything back; not only will it be futile, the truth she indirectly killed him. It was because of her that he lost his life because she made the wrong decisions.
If she wasn't so wishy-washy, then none of this would have happened.
She lied to her heart and lied to those precious to her, and that is why she lost the one person who cared the most for her.