"The ox shall be stoned" (Exodus 21:29) - why is this stated? So that one should not say, I have grounds to argue from logic: if the innocuous ox [tam, one not previously known to gore] is liable to stoning, should the forewarned ox [mu'ad, one already known to be dangerous] not all the more be liable to stoning? No! If you argued thus regarding the innocuous ox - it is because its owner does not pay the ransom, therefore it is stoned; you might say regarding the forewarned ox - since its owner does pay the ransom, therefore it should not be stoned. [To prevent this reasoning,] Scripture teaches, "the ox shall be stoned."
Why the Forewarned Ox Is Not Spared from Stoning
Yalkut Shimoni on Torah 340:9
הַשּׁוֹר יִסָּקֵל, לָמָּה נֶאֱמַר, עַד שֶׁלֹּא יֹאמַר יֵשׁ לִי בַּדִּין, וּמָה אִם הַתָּם בִּסְקִילָה, מוּעָד לֹא יְהֵא בִּסְקִילָה, לֹא, אִם אָמַרְתָּ בְּתָם שֶׁאֵינוֹ מְשַׁלֵּם אֶת הַכֹּפֶר לְפִיכָךְ הוּא בִּסְקִילָה, תֹּאמַר בְּמוּעָד שֶׁהוּא מְשַׁלֵּם אֶת הַכֹּפֶר לְפִיכָךְ לֹא יְהֵא בִּסְקִילָה, תַּלְמוּד לוֹמַר הַשּׁוֹר יִסָּקֵל.