Pride (superbia) is - according to St. Augustine - "the love of one's own excellence".
Having engaged in a couple of inter-faith debates in the past myself, I can say from experience that the danger of falling into a prideful and arrogant attitude is very real and very great.
A Christian apologist should have the same attitude as a Roman Inquisitor: to be motivated out of compassion for the heretic and to only be content with the situation when the same heretic repudiates his errors and becomes orthodox.
At times though, we see Christian apologists on a quest to "destroy" their opponents. Ridicule becomes a more frequent part of the debates. So there comes a shift in the mission: instead of "defending the truth" with the hope to persuade one's opponent to embrace the orthodox faith, the prideful apologist then seeks to "crush" his/her counter-part in order to glorify himself in his intellectual achievements: the divine mission turns to a profane one.
Thus, the apologist becomes as the prophet spoken of in Deuteronomy 18:20:
"But the prophet, who being corrupted with pride, shall speak in my name things that I did not command him to say, or in the name of strange gods, shall be slain."
Speaking of "strange gods", let us remember that the first sin was that of disobedience arising from pride: the sin of Satan. Pride can be seen as a form of idolatry as it is the "love of one's own excellence". Instead of glorifying God, one glorifies oneself. But why should one even glorify oneself? All that is good is from God. St. Augustine said that we humans own nothing aside from our sins.
Thus, let us all be humble and charitable.