Valmiki Ramayana Ayodhya Kanda Sarga 96 Shloka 24
Original Shlokaभरतस्य वधे दोषं नाहं पश्यामि राघव। पूर्वापकारिणं हत्वा न ह्यधर्मेण युज्यते।।2.96.24।।
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राघव O Rama, भरतस्य Bharta's, वधे in slaying, अहम् I, दोषम् wrong, न पश्यामि do not see, पूर्वापकारिणम् one who has done harmful act earlier, हत्वा in slaying, अधर्मेण with unrighteousness, न युज्यते is not attached.
O Rama, I do not see anything wrong in slaying Bharata. In killing one who had done harm is not an act of unrighteousness.