One day, I swore that I would do something bad but not "Haram." When I thought about what I did, I knew that I should've not said that. Do I have to make "kafarah" for what I did?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Whoever swears to do something and finds that doing a different thing is better, he should do the best of the two and expiate for his oath. It is narrated by Muslim that the Prophet Muhammad (Blessings and peace of Allah be upon him) said: "If a person has affirmed for a particular thing, and afterwards he finds something better, then he should make amends (pay Kaffara) for contravening his oath, and do the better thing". [Muslim].

Since you do not intend to do what you swore to do, you should then make an expiation for your oath.

For more details refer to: 82135

Allah knows best.