Iris was sure, she could handle this, because she knew what she was capable to do.
"Iris, you can't get involved in this. Though, I don't mind it, but unfortunately, you can't get some fun of killing them." Lou reminded Iris the same thing like Cedric did.
But, Iris knew what she was doing. She didn't need to kill them all. Her power was not limited to kill her opponent only and by her doing this, didn't mean she had resentment or anger toward them. This was only an act of defense. Nothing more, nothing less. She didn't need to feel anything about this action.
And when Lou refused to back down, Iris put the protection around her and Lil Thing, which made the merchant was unable to touch her, more so to stop her.