Is there a 成語 equivalent to "Fistula dulce canit, volucrem cum decipit auceps"?
The idea of the Latin phrase (literally "a bird-catcher plays nice songs to the birds") is that someone with unclear (possibly evil) intentions will try to attract one's subjects first with a sweet words or actions.

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Well, this famous latin expression by Dionisius Catonis I also like very much. In English it could be translated as . In chinese I usually use some wording to express that meaning, but as you want 成語, so I provide some I know:
(1)心懷叵測	
(2)笑裏藏刀	
(3)心懷鬼胎
(4)居心不良
(5)存心不良
(6)心術不正
(7)佛口蛇心
(8)口蜜腹劍

Hope it helps. Good luck.

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“心怀不轨”maybe can be used in this situation.