Sayyiduna Abu Hurayrah رضى الله تعالى عنه narrates: Verily the Messenger of Allah (peace be upon him) said: When Ramadhan comes, the doors of paradise are opened, and the doors of hell fire are locked and the Shayaateen (plural of Shaytaan – devil) are chained
(Bukhari/Muslim)
More Angels descend and praise those who are fasting.
Whilst the Shayaateen are chained and cannot lead the Muslims to sin, the evil that takes place in Ramadhan, can be attributed to base desires and influence of sins for eleven months.