Canto 1 - Boyhood
Bāla-kāṇḍa
Chapter 15: The Demigods Appeal to the Lord for Protection
Text 1.15.23

स हि देवान्सगन्धर्वान्सिद्धांश्च ऋषिसत्तमान्।
राक्षसो रावणो मूर्खो वीर्योत्सेकेन बाधते॥

sa hi devān sagandharvān siddhāṁś ca ṛṣi-sattamān
rākṣaso
rāvaṇo mūrkho vīryotsekena bādhate

saḥ hi = that; devān = the devas; sagandharvān = gandharvas; siddhān = siddhas; ca = and; ṛṣi-sattamān = the best of the ṛṣis; rākṣasaḥ = rākṣasa; rāvaṇaḥ = Rāvaṇa; mūrkhaḥ = foolish; vīrya-utsekena = proud of his prowess; bādhate = torments.

Proud of his prowess, that foolish rākṣasa torments the devas, gandharvas, siddhas and the best of the ṛṣis.

Why does Rāvaṇa need to be killed? That is spelt out here.