But these people blaspheme all that they do not understand, and they are destroyed by all that they, like unreasoning animals, understand instinctively.Blasphemous words were proceeding out of the mouths of these false teachers. And what is sad is that they were blaspheming what they didn't even understand. If they didn't get it, they they were blaspheming it. They were using their words to speak evil of all they did not understand. And their understanding came from instinct, not reason.
Animals do, pretty much, what they feel like doing. They don't reason out moralities or even risk assessment. In a sense, they go with their gut. ... or possibly, you could say that they go with their heart. I know that I would have to disagree with nearly every Disney movie ever made, but following your heart (the inner man) should not be your guide to life, and it definitely should not be the final judge for what is right for you. These false teachers were guides that were led by their instincts, by their feelings, and the result was destruction.
A truth that is found in this verse is that following your instincts (your heart or your feelings) will bring destruction. The very things these false teachers were doing by instinct is what was destroying them. The sad truth is that they probably didn't even recognize that.