处心积虑
After long deliberation.
By design.

eg：He carried out his plot after long deliberation, thinking little about others.

中英俚语俗语来源：谷梁传：“何甚乎郑伯，甚郑伯之处心积虑成于杀也。”

同义词：深思熟虑 Turn is over in one's mind.
反义词：心血来潮 On the impulse of the moment.