Actually this was the motive of there incarnation to create a fake religion. In kaliyug, asuras started following Vedas but still they spread terror. As they were following path of Vedas, Deva couldn't defeat them. That's why Vishnu incarnated as budha to take them away from Vedas by making another religion.
Padma Purana VI.252.108-115
“…The glorious one, taking up the form of Buddha, deluded his enemies. Those who were deluded by that teaching gave up all religious practices….” Tr. N.A. Deshpande
Srimad Bhagavatam 1.3.24
“Then, in the beginning of Kali-yuga, the Lord will appear as Lord Buddha, the son of Añjanā, in the province of Gayā, just for the purpose of deluding those who are envious of the faithful theist.” Tr. Swami Prabhupada
Garuda Purana 3.15.26
“Then in Kali age, the Lord was born in the Kikata [modern Bihar] as Buddha. He deluded the asuras and flouted the Vedas.” Tr. Board of Scholars, edited by J.L. Shastri
Srimad Bhagavatam 2.7.37
“Asuras, the enemies of the celestials, became proficient in the knowledge of the Vedas, and having got made by Mayadanaba (an Asura of that name) and invisible city of velocity, engaged themselves in the destruction of the people. Thereupon the glorious God incarnated Himself as Buddha, and with a view to bring about a confusion of their understanding and to create avarice in them taught them many false religions in the guise of a Pashanda (one setting the true religious texts at naught).” Tr. J.M. Sanyal
Agni Purana 16.1-5
“Agni said:- I will now describe the Buddha incarnation…Formerly in the war between gods and demons the former were defeated by the latter. Saying ‘Save us, Save us!’ they sought refuge with Iswara. Being incarnation of illusion and infatuation he was born as the son of Suddhodana. He infatuated the Daityas and made them relinquish the religion of the Veda. They then became followers of Buddha and induced others to forsake Vedic religion. He became Arhata and afterwards made others Arhatas. Divorced from Vedic religion they became all Pashandinas [Irreligious people]. They committed (sinful) deeds capable of taking one to hell and received (gifts) from degraded persons…” Tr. M.N. Dutt
Note:- it did not mean that all budhists are demon.