But Ittai answered the king, “As the Lord lives, and as my lord the king lives, wherever my lord the king shall be, whether for death or for life, there also will your servant be.”
Ittai is legit. David tried to give him an out previously in vs. 20, but rather than take it, it only strengthened Ittai's resolve. Much like when Jesus asked, "Do you want to go away as well?” to which Peter replied, "Where else would we go? We're with You."
Ittai was a man who chose his love and then loved his choice. He would rather die as David's servant than live as his own king.